I don't think that's what it's for.  I think it's mostly a way to reply to a person without quoting their entire post (as sometimes you just want to say 'I agree' or 'well-said', and nothing more).  It lets you know when someone replies directly to you in a discussion, which can really be helpful because sometimes I forget that I've even posted in certain threads so never get back to them to see if someone replied to me. 

das