The most common use cases for me are just replacing words and tweaking alignment. Obviously there are ways to do it in other editors, but having multiple cursors exposes these capabilities in a convenient way.

But one thing it's really useful for is making bulk replacements on the fly without having to think too hard about what to match on. Instead of fiddling around trying to create a perfect regex to match the right cases, you can just step through instances of a loose match, choosing the right ones, and then make your edit.

Overall, for me it just feels like a much more intuitive ad-hoc style of editing.