That stuff is fun in school. Nobody knows anything about it in real life, yet everyone gets by...
Indeed, I'm developping a project with some other people spread all over the country and our UML diagrams make sure everybody working on the project has a clear view of what we are making.
Also it helps devide the work to know who is working on witch part. Ones you have the UML diagram coding it takes less time... And even maintaining your code is easy for everybody who can read the diagram.