Part of it is keep away but I don't believe that's the primary reason. Britton replaces Chasen Shreve as a far more competent lefty specialist. Brian Cashman said earlier in the season that because the roster had many desirable prospects on it, he would look to trade some of it before the season was done. This is because the Yankees can only protect a limited amount of players when the Rule 5 Draft takes place after the season. Yanks would have been losing prospects like the guys they traded away, so this "bundling" of guys helps to mitigate that AND get a quality guy in return. He can be a difference maker. He's also another option out of the bullpen to close games IF Aroldis Chapman has lingering problems.