If you can use something, it's useful. Useful things are handy for whatever you're doing.
If something is just what you need or you just find it helpful, then it's useful. A hammer is useful in building many things. Coasters are useful if you don't want a coffee table to get stains. College degrees are useful — or necessary — for many careers. MacGyver would tell you that just about anything can be useful at some point: it all depends what you need to do.