You can see the comms that the user is a member of on the profile. (Mostly.) You can see the users that a comm is a member of. Sometimes a user has specialty interests and bio that they want to show to other comm members but don't necessarily think is worth/for public consumption of their regular profile.
Like, where I intersect with Vorkosigan comms, I want to list that I was a member of the List since 1997 or thereabouts, maybe 1996, and that I sorta drifted away but I'm still reading and happy and alive and all. And all sorts of fannish details that aren't relevant to the main part of my life as expressed on LJ.
It would be so damn nifty (and such a fucking pain for the user to keep updated, and such a fucking technical pain probably) to be able to enter a separate bio and interests for each place I intersect a community.
I don't know if I would want the bio/interests visible to non-members (and here we run into security issues, because if it's an open comm, well...) (but then some open comms ask for member introductions, and yes you have to search for them but) but again you could do the link-to-locked-entry (or in this case members-only entry) dodge if it's public.
And it would be simply ace to be able to do interests searches and search from the interests of either/both the general public, and the community-specific interests of people in the same communities as you. You already have at least one point of contact.
OK, people, tear it to shreds. I want it plausible.