It usually involves developing a design language for the app, or sometimes, for the whole organization (if, like the one I do a lot of work for, it's really all about one app). That's a big deal.
Logo design is also a much more difficult task than people think. A good logo can be insanely valuable. The one we use for the app I've done a lot of work on, was a quick "one-off," by a guy who ended up running design for a major software house. It was a princely gift.