You could, to be sure, make them self-contained. No event in one book impacts the events of another. Characters who reappear are static, and there from the beginning. This can get dissatisfying, monotonous, and implausible.
Suppose, on the other hand, the events in one book do affect the next.
If you stick to the same cast of characters, either the series will end because the characters have developed through all their problems, the series will limp on with characters who have completed their arcs, or the story will effectively turn into the self-contained books because while they make changes, they do not make effective changes, leading an unrelenting assault of problems. Similar effects with settings or magics or events or things.
When you add more stuff, you soon find the world getting larger and larger. And stuff that logically would reappear doesn't because it doesn't get remembered, or it would make the story too large. And even at the same time, characters reappear for bit parts with varying degrees of elegance, but no room on the front stage. And backstory gets more and more complicated. (Gets complicated with the same cast too, but less so.)
Jim Butcher's Changes -- well, the first thing I noticed about it was when I first heard it was coming out. The title is not one of the two word paronomasia (or pun o:) that he's been using in the last twelve books. And he made changes all right. Several major plots have had a fork stuck in them, but good. My own suspicion is that he wanted to clear the stage, which had gotten too cluttered. Sure, all the stuff floating about could be fun, lending solidity to the world, but even before this point, I had wondered about stuff getting dropped when I thought it would come it.