“A series can be great for authors because it can draw in readers and keep them. If they like your first book and its characters, they’re likely to forge ahead and buy more books in the series. This is why there are so many series out there.”
Very helpful post for authors, beginning with whether there’s one plot that continues throughout all the books or whether a character or a setting remains the same while the plots change.
When done well, a series will make for a continuing use of a great plot or a great protagonist and engage readers for (possibly) many years.
There’s a lot to consider, and R. J. Crayton does a great job with this thorough post.