The Spanish Army has a long history of using trucks to move tanks to and from battles -- in fact, they commonly used trucks to move tanks everywhere the tanks had to go. They did this to save wear and tear on the tanks as these pre-WWII tanks had a very short maintainence cycle. Because it was a common procedure, and there were even incidents of tanks fighting from the beds of trucks, the ability to carry tanks with trucks has been included in this OOB.*
*
Note that the Soviets in Spain quickly decided to follow suit and truck their T-26's around once the T-26's started breaking down from being overused -- after the first two months of service!