Earlier on in the start-up's life the functionality was introduced to send replies (via FriendFeed) to other people's Twitter messages. This worked as long as you had plugged in your Twitter credentials to give the app permission to post as you.
This new system is similar to that model, although it can be set to do this for everything you publish to FriendFeed. Users can also select which specific services they'd like cross posted, keeping your Twitter subscribers from being inundated with your comments about threads they have no idea about.
To avoid what could be considered an infinite feedback loop, FriendFeed ignores these Twitter messages, even if you've set up your Twitter account to go into your FriendFeed. If other Twitter users reply to you it will simply go into the original FriendFeed entry.
This could mark the start of export publishing to other services, which VentureBeat's MG Siegler thinks could be a simple play to drive more traffic to FriendFeed through back linking. As it stands any message sent to Twitter from FriendFeed includes a link back to that conversation, which might be enough to hook people.
Personally I really don't feel the need to use this feature--at least for Twitter. I think Twitter's strong suit is that it's text only, and while links are sometimes handy I don't think my followers want as many as FriendFeed is likely to spit out. FriendFeed does a far better job integrating photos, videos and other media, which is something I doubt Twitter is likely to add; that is unless it develops a plug-in architecture (which the recently updated side-bar suggests). For the sake of my followers I won't do this.
Where I can see some value in this is for something like Delicious. The tools for sorting through Delicious bookmarks are plentiful and powerful, but lately most of my link sharing has been to FriendFeed because I prefer it's media-rich bookmarklet. Sure, I could keep adding bookmarks to Delicious, then have them get sucked up in FriendFeed, but I know for a fact that my FriendFeed subscribers would probably be more likely to check out that link, or like it if it had a picture and a snippet of text. With a system in place to push out FriendFeed bookmarks to Delicious, I could avoid this problem altogether, and still keep my Delicious library fresh.
I expect we'll see other services for export publishing in the near future--the question is whether it's more for the users or FriendFeed's traffic.