Jet2 have obviously identified a need for more capacity in certain routes during the summer. Given their cautious expansion over the past few years, they wouldn't switch from Boeing to Airbus and from narrow to wide body without a proper business case for doing so. I think a damp lease is very sensible, allowing Jet2's cabin crew to continue providing the Jet2 product while not having any flight deck or engineering responsibilities for a brand new type to the operation.
I would imagine having the ability to sub-lease it to others over the winter for long haul ops and/or to use it themselves for a slightly expanded winter long haul program (ie. the NYC trips or possibly Orlando) is part of the reason it's been decided to add it to the fleet. And if it doesn't work out for whatever reason, being leased, it can always be returned at the end of lease and the program can continue pretty much as it has in the past.