It wasn't just a German thing: tanks have weak spots because armor isn't uniformly thick around the tank.

If a tank is expected to move forward and fight other tanks, it's going to prioritize having more armor in the front and less on the sides and even less in the rear.

Likewise, tanks will prioritize armor against other tanks, vice armor above - which makes them more vulnerable to air attack (or in modern days, anti-tank missiles that come from above).

Not to mention, a tank is more likely to carry its ammunition and engine in the rear part of the tank, making direct hits there more likely to disable or destroy the tank outright.

Using The Collins/Jane's Tanks of World War II, the American M4 Sherman, for instance, had a front turret thickness of 76.2mm at a 30 degree angle, giving it an effective 87.9mm of thickness. In the sides, it was 50.8mm thick with a 5 degree angle from the vertical. In the rear, it was 50.8mm thick and pure vertical, while the top armor was 25.4mm thick and flat.