I can find not only the definition of variables and functions (which ctags lets me do, too) but also such nice tidbits as "who calls this function" and "which functions does this one call". Especially the "who calls this function" and "assignments to this variable" are very useful when you're trying to find out how things come to be.
Edit 18:40: And thank goodness for ctags, too. And vim's ability to keep a tag stack. And the ability to double-click on an identifier to be taken to its definition. My word, these things are useful. (And I think using ctags is a bit faster than cscope so I tend to use tags when it's something that ctags can answer.)
I am so glad I don't have to sit in front of a stack of fan-fold program listings with paper clips in strategic locations.
Thank you, Bram Moolenaar! Thank you, Darren Hiebert! Thank you, all the others who work on such lovely tools!