In fact, the main reason for adding muzzle breaks WAS the recoil. The Tiger I's m.b. eliminated about 70% of the recoil force! It helped to keep the size of the turrest small. Usually the gun would be damaged when fired without the muzzle break as it hit the rear of the turret. In tank destroyers there was usually more room the muzzle break was not always necessary. One reason why the Jagdtiger didn't have a m.b. was that the gun itself was rather heavy which also reduced the recoil. You will see that some naval guns have massive breech blocks to add weight.
If it was for better visibility or not depended on the construction of the m.b. Sometimes it was better with, sometimes better without a muzzle break. But when a heavy gun is fired, there is always a lot of dust!
