20mm and 37mm flak cannons are incredibly effective at taking out enemy gun emplacements. If I'm attacking as the Axis, I like to have two 20mm towed by kubelwagens that can be sent in quickly to take out enemy anti-tank assets. I once had a single regular 20mm take out three 75mm anti-tank guns from three different positions that they were deployed to as I advanced. Used in that way it's like a super-machine-gun of some sort, and has a really easy time keeping gun crews pinned.