The fact that ASOIAF's plot is basically an escalating series of butterflies probably helps as well.While I can't quite put how into words, this reminds me of the discussion about why more people do AH of Westeros on Ah.com than other fictional worlds (I think @zaffre was involved?) and how it having more long-term (if that's the right phrase) and wider-ranging world-building is part of that and that in turn in part being down to having rather more POVs.
I suppose spinoffs (if done well) are both the product and a cause of multiple POVs and more extensive world-building, in what can hopefully be a virtuous circle.
Having them available is a reassurance that there's substance in the background. Whether or not anything gets done with them (by you or someone else) is a secondary issue.*Glances nervously at giant list of potential Broken Lance spin-offs and the fact that I've only managed to crank out one and a half of them with three months of free time*