The Jets got nothing out of the slot last season. David Clowney's productivity ended when the preseason ended. Brad Smith asserted himself as a useful player in a number of spots. One of them was not, however, wide receiver. Wallace Wright made some strides in workouts and practices but didn't impress the coaching staff enough to see significant action on the field. This is part of the reason you have seen so much talk here about taking a guy like Jordan Shipley in the Draft.
All of this time, the Jets had an excellent slot receiver on the roster. He is a guy with excellent hands who runs routes with pinpoint precision. He is good running after the catch. His name is Jerricho Cotchery. The Jets couldn't play him at the slot, however, because they needed him on the outside.
The Santonio Holmes trade will allow Cotchery to move inside in three receiver sets. The Jets now have a pair of receivers good enough that they can stick Cotchery inside without losing too much on the outside. It's the hidden benefit of the deal.