It's the replacement of BOMArchiveHelper in Leopard. It opens up whenever you open an archive like a .zip file and extracts its contents.
oh right!
There it is, lurking at the bottom of Apps. Never used it, been using lots of other 3rd party applets for the obscure stuff.
I'm on 10.4.11 for ref, dunno when this little guy turned up.
When I look at the prefs, there's a huge list of basically any and every type of archive you can think of including 7zips
That'll be why I'd never heard of it then...