useful

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.