A tank's tracks (the individual pieces that make up the treads) are neccessary for movement. These tracks are linked together with track pins, and in the course of combat and movement the pins slowly wear out and might break. The loss of a few tracks would essentially make the tank useless - unless of course the tank has a few spares.

Both the Soviet Union and Germany equipped their tanks with spare tracks, which were stored on the outside of the tank. The spare tracks could thus also act as extraa armor. Spare tracks enabled the tanks of WWII to solve the problem of broken tracks even without a friendly motor-pool or engineer brigade, the lack of which would otherwise render the tank useless.

WHERE TO LOOK FOR SPARE TRACKS

Spare tracks were routinely jerry rigged onto whatever part of the tank the crew deemed fit. This normally included the front faces of the tank as well as the sides of the turret.