I was trying to puzzle out the structure of a few PHP projects and I couldn't make much sense of them considering they had a whole bunch of classes, functions, methods, etc. So I thought to myself, "hey, a function/method call graph would help!" I didn't want to profile the code and see what kind of performance it has, all I wanted to know was what kind of functions/classes it uses. This is useful because it allows for a quick glance to evaluate how well the code is written. If a single function uses 20-30 different functions inside itself, it might need refactoring. Or when you get a stack trace from PHP, it might be helpful to see what's calling the function that failed and then to compare it to other calls made to it.
You can check out the project here or on Gitorious. I'm going to try and submit it to PEAR as well.