Adam Clarke on 1 Samuel 21:6

So the priest gave him hallowed bread - To this history our Lord alludes, Mar 2:25, in order to show that in cases of absolute necessity a breach of the ritual law was no sin. It was lawful for the priests only to eat the shew-bread; but David and his companions were starving, no other bread could be had at the time, and therefore he and his companions ate of it without sin.