Reserve soldiers for a tank

What is the purpose of having the extra two soldiers on a tank crew?

Each tank crew division can drive many different vehicles. Some require 3 crew members, some 4, and some 5. Once you unlock things like Shermans, Panzer 3 or 4s you will be able to use all of the tank crew. (tho 2 of them still don’t really do anything, radio operator and loader)

Understood, thanks! I didn’t think about upgrading vehicles.