Very frequently airlines wet lease aircraft from other airlines to operate routes on their behalf.
The airline that takes the wet leased aircraft takes all the financial risk for slots, revenue, fuel, handling, marketing, etc whilst the airline that provides the aircraft provides the maintenance, staff etc.
In this way, an airline could launch a new route with a wet leased aircraft, and not worry about commonality - but pay a hefty sum to the leasing airline for the pleasure. It could be an interesting way for a smaller airline to fly try one or two larger routes without bankrupting himself with the commonality problems, or for a large airline to have a micro fleet to fly specific routes or provide an interim solution.