Personally, I tend to find the "blank slate" thing daunting, and compensate by clinging to any scrap of canon about a minor character.

But as for motives, you can add "sheer perversity" *g*. Obviously enough, we only tend to be shown secondary characters in terms of their relationships to the core characters, and it's interesting to reverse that, to try making them the centres of their own stories. And it can be a way to explore under-developed aspects of canon, or to look at the canonical world from a different angle.