From a reading perspective, getting into a series is great. Once you’re familiar with the set up of characters and the backdrop their tales are told against, reading a new book in the series is like curling up in a comfortable chair, the fire lit and a glass of your favorite right to hand. Time for a good read. Yes, it’s that comfortable. You know in advance it’s going to be good. You already know the background, you don’t have to learn it. It’s easy to slip into the right mindset, the right place in the story and find out what your favorite characters are up to this time, or even meet new ones.
Writing a series is good, too, pretty much for the same reasons. When the galaxy has been sorted out (I write science fiction) and you know who’s who, writing is faster and easier because the backdrop is already done, already there, already wonderful. The politics are sorted, the structure, the scope.