Contracting out the regional operations is principally a way of reducing costs and risk. UX staff (especially pilots) don't get paid as well as UA staff, and UA has limited risk exposure on those flights since the UX partner gets a flat fee per flight.

AA, DL, and NW all own part of their regional operations. DL is trying to spin off Comair and sold ASA, while NW has been moving in the other direction by starting Compass and "adopting" Mesaba. Go figure.