There are a few main reasons someone sets out to retrieve past life memories: phobias unexplained by anything in their lifetime; a relationship that has proven destructive which they don't seem able to leave; unexplained health problems; a driving impulse that leads them to follow it; or a strong curiosity about what dying really means.
Regressions take a lot of work and are not fun whether they actually tap into the past or are simply the subconscious giving allegories to illustrate problems. I can see how they would probably be a mistake for someone whose personal world is working well. Why look into past lifetimes to find problems you don't currently have? Why stir a pot that isn't boiling?
One of the first books I read on the subject of reincarnation and regressions was by Dr. Brian Weiss, a psychiatrist, who was using hypnotism to help a patient recover the events that led to her current problems except she went farther back than he intended and into past life memories. The experience, as well as those that followed with future patients, led to a turn around in his belief about reincarnation. He came to believe not only that reincarnation was real but that understanding past lives could lead to solutions for current problems. He wrote a series of books on his experiences. He said 'coming out' was not an easy choice as he felt he'd be regarded as a wacko-- something no doctor wants to be thought.
Where regressions seem to be most helpful (with an experienced therapist), is where a person has problems that other methods haven't touched. For instance it's been found people with severe obesity (that doesn't mean need to lose some weight to fit societal norms) have sometimes found the reasons lie in past lifetimes of starvation. Some current health problems have been helped by going back to a lifetime where something explains it and the person can then release it. Some in very detrimental relationships realize it's a pattern repeating again and again and give up trying to fix the connection and instead concentrate on safely getting out. Perhaps regressions could be thought of as a last resort, not a first, and certainly no magic panacea.
As some mentioned in comments (which for me were particularly interesting this time), I have read that there are old and new souls. Why would we assume creation, even if reincarnation is part of it, had to end with the original event? Some of my friends have been told this would be their last incarnation.
And yes, there do seem to be souls living more than one life at the same time. One of the books on regressions by Dick Sutphen dealt with his wife finding another lifetime her soul was leading now-- one totally different from what she herself was living. She hadn't, at the time the book came out, tried to meet that woman. (Sutphen is one who has written a lot of books, produced of the tapes I used, as well as does seminars where he has regressed groups at the same time.)
To add to the mix in comments, parallel universes were mentioned and some do claim they have traveled between them. There are things in this world that are hard to explain. So much about the energy of life we barely understand or have touched on.
While I mostly did my regressions solo, I was later told by regressionists and psychics that working by myself was risky but basically those kinds of tapes simply won't take you to a level to do true work in areas that might need the help of a therapist. They touch on areas, don't delve deeply into them. Although even mine which didn't go deep had some tears and upset attached to them during the meditative sessions. One, as an unfaithful wife, had my nose tingling when it came time for the consequences (Apaches had the option of cutting of the noses of unfaithful wives), but I saw no nose cutting-- fortunately.
In my one session with a hypnotherapist, she told me a story about one of her own experiences that can explain how this process can help if used properly. Her daughter was very clingy and constantly didn't want her going out of her sight. The woman eventually found the past life that led to that when she and her daughter were twins and she died young leaving her twin feeling deserted and leading to the fear in this lifetime of a repeat. It gave her understanding to deal with it this time.
There are a lot of interesting stories if you begin to read the books or even talk to people about the experiences they might hesitate to tell for fear of being thought crazy or not believed. Some people retrieve a memory and have confirmed it by traveling to where the events happened.
One of my questions about reincarnation has been-- if we are all coming back, why isn't the world getting better? And my brother said-- maybe we are the ones who just goofed it up, didn't get it right, and this planet is full of losers. He was joking, but sometimes it'd be easy to believe that.
Another problem with past life memories is how do you account for so many people being say Mary Magdalene? One explanation I read suggested perhaps those, who were nearby when these things happened, confused what they saw with who they were. Another possibility would be group reincarnation where you'd have parts of memories of many people. I can't explain it but definitely too many people were Thomas Jefferson and nobody admitting to Hitler... yet. (Coincidentally, relating to this very topic, I came across this article in the LA Times-- Marilyn Monroe is reincarnated as this woman?
I don't have answers about these things, but have found it interesting to explore and my personal experiences did explain some things for me. But as I said from the beginning, I am still not sure as not only are my dreams often in story form, but I write fiction as well as create art. I could have created anything I got-- although my creating a story doesn't have me in tears as the regressions did-- not to say they couldn't. The only way I could probably 'know' would be to go where the stories happened and see if I felt I had been there before or if a friend had gotten the same memories of some past life event but from a different perspective. And even that can be explained away if a person doubts.
One more thing, before I leave this topic for awhile, is astrology can pick up on past lifetimes without the trauma of actual memory retrieval. If you are curious but don't want to put the time into the regressions, find a 'good' astrologer who does this kind of work to look at your chart. And some astrologers can also find the geographic location of those lives.
If you are willing to work for the information yourself, a useful book is 'The Elements of Astrology' by A.T. Mann (good on explaining reincarnation also). Through his graph and your natal (birth) chart, you can look at when you might have had previous lives that are impacting your current life (not all lives, just those that relate to who you are this time).
Other books dealing with karmic relationships can show you (if you have another person's chart) what kind of relationships you might have had with them in past lives. Yeah, I know-- might-- but if you don't take it too seriously and have fun with it, it can be interesting. I came up with an interesting story (unwritten as yet) through my own chart and that of some friends where we all may have had a lifetime in Mongolia long before the time of Genghis Khan (according only to astrology, as other than some possible... er personality traits, I have never retrieved memories from that time).
I have never had a fear of death (dying process worries me some having seen old age up close with our families). Dying itself might be a bit scary but also interesting. To me death of the body is a transition and the mystery is to what do we go? When my mother died at 85, she was in her own bed, didn't know she was seriously ill. One of the things I thought afterward was-- now she 'knows.' Someday I will too.