Protecting yourself against German air attacks is crucial to keeping your army in tact. AKA - You've gotta deal with those Fokkers before they become an issue. For the Soviet army, that came sometimes in the form of the GAZ truck mounted simply with four Maxim AAA guns attached to a stand in the back.
The big water-cooled Maxim guns were heavy, and having four of them attached to a single mount meant they couldn't be moved very easily. Pretty much you had to set them in place or put them on the back of a big truck. The recoil from all those guns firing at once could amount to a rather sizable force. But hey, they were relatively cheap, effective, and easy to produce in large numbers.