I find it useful for code reviews (spawn a subagent with minimal/no context to review X commit). Of course, this is more or less a shortcut that could be done with a seperate agent. Another use is multiple reviews at once if tokens are not an issue, with seperate "personas" or focuses. As far as implementation goes I have not seen any major usecase.
Yeah, my personal workflow has different reviewers for codebase(patterns, code cleanliness, etc), frontend, security, product fit, etc. So they spawn as separate subagents. Both so that they stay limited to their role, and so they don't have preconceived notions about the implementations. It's a bit heavy-handed but works for me.