When starting out writing there's an early question that must be answered: will the first book be a stand-alone, self-contained story or the initial part in a series?
Fantasy is perhaps renowned for its series, some of which are very sizeable. Lord of the Rings stands out, as does Wheel of Time, A Song of Ice and Fire, and The First Law Trilogy.
A series, whether a trilogy or a longer series, enables the writer to use a more detailed world and larger cast to tell a deeper and longer story. Which sounds cool, although there are certain pitfalls. For a start, it's possible to write a trilogy all at once and release it together (to help promote sales of book 2 and 3, as people who have to wait a year or two for the second part may neglect to buy it) but that does involve a long delay to publication and cannot really be done for a mega-series.
There's also the problem of each book being worthwhile in itself. By definition, a stand-alone novel has to stand or fall on its own merit. With a series, there can be a danger of one book being more about tying up loose ends from its predecessor and setting up the plot of its successor rather than telling its own story.
Variable quality's another issue. If you've bought 7 books in a series, but know from the reviews of people you trust book 8 is a bit poor, you're faced with the unpalatable choice of ditching a series of which you've read thousands of pages or forcing yourself to endure gruel whilst waiting a year or more for the tastier next course.
So, given that, why would anyone prefer writing a series, long or short, over stand-alone books?
Simply this: a series means characters can be developed more, and have more complex relationships with others. Plots can be more intricate and numerous, the world described in greater detail and the emotional investment readers have can be increased significantly. In addition, some stories are just too large to confine to a single book without making it a massive tome.
It's not entirely an either/or choice, though. Lots of authors choose to write stand-alone books set in the same world (such as Joe Abercrombie or Scott Lynch) or trilogies set in the same world (Robin Hobb). This means the reader gets a nice sense of continuity and familiarity, and the author gets to explore and expand an already existing world without having to create a new one for every new book.
For Bane of Souls, and Journey to Altmortis (due out next year), I went for the stand-alone approach for a few reasons. It's much quicker than writing a series (if I chose to write a trilogy all at once I'd be about half done now), and both of the stories were enough for a single book rather than needing several. The trilogy which will follow Altmortis will be of lengthier proportions because the story, a civil war, necessitates it.
Not sure what I'll do after that (it'll take me quite long enough to get it finished) but I'm near-certain I'll never write a mega-series. They take too long, readers can understandably get frustrated if there's a prolonged wait between instalments, and I don't want to be tied down for a decade to a series.