I think too much novelty is as limiting as too little. Imagine TV where every programme is an original one-off play. It'd be like never having anything longer than a short story, and I know I get fatigued by the constant stop-starting of a set of short stories.
Series characters give us a shorthand. We already know these people, this world. We can get on with the new story without having to set everything up again. Also we'll often like these people, and want to know what happens to them next.
It's just playing with different lengths of form. You can tell a shorter story with a longer underlying arc. It can be a lazy cash-in on some beloved characters, but it doesn't have to be. It's just another story-telling tool, and the more the better.