Until then we had been bombing only military targets. The Americans bombed during the day in order to be able to accurately hit legitimate targets, while the British bombed at night when it was safer for the pilots, but less accurate.
I think the British just did it because they were mad at the bombing of London. Still, in that one way they lowered themselves to Hitler's level and even below. It actually hurt the war effort in the sense that it gave Hitler a massive propaganda piece to rally the beaten populace against the Allies.