Well pretty much anything you'd not be comfortable being in your bookmarks. Maybe you're a closet gay, atheist or have a weird/embarrassing condition that you're researching and don't want those links saved in your bookmarks.

And the biggest use case - read later! Using bookmarks is a horrible way to save stuff you want to read later. Often times I even forget I had something bookmarked to read later. A nice browser extension that shows if you have any links in the queue. You could probably build on of the xstashed code, and turn it into something more innocuous - maybe ystashed