Thread:Translating talk:MediaWiki/Helpful vs. Useful/reply

They are. I don't know if they're even used consistently in the interface. However, what they actually mean with "Helpful" is probably "Non disruptive" (e.g. not "asdasdadasdadsasadasds"), while "Useful" is a feedback that is also actionable, that helps improving the page (e.g. not "the article would be improved by a photo of Caesar": true, but impossible).