It's just user-generated content, meant to give some replay value. It's Hitman's version of multiplayer more or less.
Besides that, not much of a point. You can earn "cash" that you can use to buy unlocks (disguises, weapons, etc.) but I believe you can get them all in the campaign.
You can also buy upgrades for your weapons, but from what I can tell so far it is highly optional to upgrade any weapon. I can't say I've played on Expert or Purist, but I can't imagine it's absolutely necessary to play with upgraded weapons.
Oh, and there are a few trophies associated with it. I think that's it.