I think the idea is to track discussions through email as threads, rather than just a long list of messages.
That said, you bring up a valid point about reinventing the wheel -- GMail does this already with conversations. So this app would need something more.
One idea off the top of my head is to enable people to participate in a discussion using an account on the site, rather than through email. Maybe also allowing people to comment on public discussions, but giving their comments a lower priority (along the lines of HN's prioritizing of comments).
That said, you bring up a valid point about reinventing the wheel -- GMail does this already with conversations. So this app would need something more.
One idea off the top of my head is to enable people to participate in a discussion using an account on the site, rather than through email. Maybe also allowing people to comment on public discussions, but giving their comments a lower priority (along the lines of HN's prioritizing of comments).