Essentially for all those questions that get bumped from the currently existing sites for being in the wrong place. E.g. if a perfectly good and interesting question has no natural home in the existing family of 'stack' sites it could 'fall off the end' and find a home at an 'odds and ends' catch-all site. A sort of SO reject bin if you like.
You could call it 'orphaned storage'.
Periodically the tag stats could be analysed to sift for promising new stack sites.