Past continuous (was/were + -ing) sets an action in progress; past simple reports a completed event: While we were packing, the organiser called. When does not automatically require one tense: meaning decides. An -ed adjective describes a feeling (interested), while -ing describes its cause (interesting). Useful is helpful; useless is not effective, not morally worthless.
Topic preview: Volunteering can build patience and teamwork when the work meets a genuine need. At a reading club, a volunteer learns to adapt an explanation to the reader rather than repeat it more loudly. However, goodwill does not replace training or safeguarding. A supervised, regular commitment is more useful than promising many hours and then disappearing.