They don't need the roster space. They now have 66 players under contract and are allowed a maximum of 90, which usually fills out after the draft.
With the draft still three weeks away and facilities locked up, there are no offseason programs where, say, the other two quarterbacks need to take all the reps in practice.
Could the Patriots have done Kessler a solid by releasing him now to give him a chance at signing with another team before the draft? Sure, maybe.
There isn't much of a salary-cap benefit to the move. Kessler was making $935,000, but he's going to need to be replaced in the Top 51 with a salary that counts $675,000 so the net is $260,000. That leaves the Patriots with just over a million in cap space at the moment, so effectively now they could sign a free agent with four to six years service time and be under the cap.