The short explanation is that it defends against rockets. For the longer explanation, you have to understand how such rockets work. They don't just hit the target and explode, making a big hole. This video demonstrates how a modern RPG-7 rocket words. That weapon, being based off the German WW2 panzerfaust, is very similar in how it works.

Basically(since the video itself is boring) it used a shaped charge to concentrate the explosion into a small point, cutting through the armor. It basically acts like a blow torch on steroids. But if the rocket hits something such as a metal cage or, in this case, a log, the charge will go off and only cut through that. Not letting it cut all the way through the armor itself.

The logs were just something field expedient since the military didn't issue such defenses back then. Basically anything could work. From sandbags, to logs, to metal caging welded onto the armor. You can find all kinds of pictures of WW2 era tanks covered in extra armor by the crew.

They're for un-ditching the tank. If it drives into a ditch, or onto a stump, you can wedge a log into the track. As the track moves round, the log will push the tank off the obstacle.