helpful for doing or achieving somethingHere’s some useful information about travel in Canada.
a useful tool/technique/gadget
Her book is a useful contribution to our understanding of how Mozart worked.
useful to: I was useful to them because I could speak French.
useful for: That basket would be useful for picnics.
useful for doing something: Old pictures are useful for seeing how people used to dress.
useful (for someone) to do something: Thanks. It’s useful to know that.
Would it be useful for you to ask us some questions?
prove useful: Keep a record of everything that might prove useful.