To be useful to have valuable utility or resource. But being useful is subjective.
Everyone has their own ideas about what usefulness is. One may feel it's about selfless acts that help others. Generosity, philanthropy etc. Yet others might place emphasis on doing the 'right' thing.
There are other ways to be useful too and this doesn't encompass the above either. In some cases, doing the right thing or being helpful can still be useless.