Good Q. I think in future each community would be tied to a different SMT, so it would make more sense then. Aside from that, if you want to pull a post into your community that was posted outside of it, this would be the mechanism. If you weren't following the original poster and came directly to steem for a community, you might not otherwise know that post exists.

Beyond that I'm trying to figure out the implications myself. I did just discover that crossposting will draw the autovotes following your account.