What a series does is when a reader likes a set of characters, a place, a theme, they often will follow it up. I got one criticism early on that i hadn't made it clear that the book they bought was second in the series. Readers like to read books, especially with connecting characters, in order. When you do an ad, a series makes a reader more likely to also buy the next and the next. I've seen it play out time and again. You pay to advertise one book but reap the benefit for the rest.
Series can based on a place, characters, or theme. You have seen how that worked with wildly successful books, like the Harry Potter books, Game of Thrones, etc., but it's a plus even for those that are not.