There are so many websites (and in particular, web applications) where the vendor got a minor thing wrong. With Stylus and a bit of quality time with the Firefox Inspector, many of those issues can be easily papered over. And that's more than cosmetics: a password vault that uses fonts that mix up small L and digit one is a hazard, and an application where a tighter linespacing removes an unnecessary nested scrollbar becomes just a bit more productive. In short, Stylus is ideal whenever you can't just edit the CSS behind an application.