In the wonderful world of Fantasy, it can be daunting to put the complex lives of your characters into the constraints of only one novel. A lot of authors have sought out sequels and series to fully examine their tales, some even, years afterwards find out that there was more to the story and write up sequels or prequels to their tales.
The when and why questions that come up for an author can only be determined by the author. When should a novel actually be two novels? Why should this story be explored through a series of books? Creating a series can enhance your story but it also can water it down, drag it out, make it unenjoyable by your reader. I think a good general rule is length (if it's a long story maybe it can be cut in two) but it's really about what the author thinks, if their story is complete and yet there is more to be said (enough to fill another novel), then maybe they'll begin anew, if there is no more to be said, or too little, they may just abandon a sequel.
Some more memorable Fantasy series are:
The Wheel of Time Series by Robert Jordan
Mists of Avalon Series spearheaded by Marion Zimmer Bradley (she wrote/co-wrote most of them).
Choose Your Own Adventure Series by Various Authors (I know this series is fantasy and sci-fi, but no one gives it props anymore):
As a reader, you'll have a preference for series or for single novels. Do you like to get attached to characters and follow them through their lives and through their children's lives? Do you like a quick read that is over after the first novel? My favorite books tend to come from series, but is that because they are in a series, is it just a coincidence or is it because I get to choose my own adventure?