I don't understand why anyone with the willpower to grind out a story would write it about someone else's setting, characters, etc. It baffles me.
Speaking not as a ficcer (despite a vague and reoccurring interest (why so terrible, motivation?)) but as someone who knows and talks with one (reasonably decent, I believe) fairly regulardly, various reasons, some of which have been covered.
In this particular case, for this particular fandom (Golden Sun, if anyone cares), it's primarily as exploration, because the games leave
so much unstated; enough to give an idea of what the characters, the world, and such are like... but leaving
so much more to the imagination. You have a base; what comes after it? What else is there to the characters that the sources only hint at? What else is there in the world that is never seen? There's also practice (you can't become a better writer without actually
writing), experimentation with new writing styles, a way to pass the time, or just ways to explore random plot-bunnies that won't get out of your head.
Reasons vary. Some things work better with some settings than others. Sometimes it's just to explore possibilities (what if X happened instead?), sometimes just to do more with the characters one likes (or hates (or just didn't get enough development)), or sometimes... well, ask the writer.
In more HS related news, referring back an update, someone erroneously explained Scratch's reaction as not wanting to see Slick PASSIONATELY HATE-SMOOCHING his ectobiological mother. I mention this because people (including myself) misread the assertion as meaning Snowman was
Slick's ecto-mother rather than Scratch's (which she isn't on account of simply being... an instigator, I guess, but not quite mother). I mention this only to put the idea in your mind.
It wouldn't be the strangest thing in the comic.