Trucks and APCs are units of logistic support. Some versions also are fitted with machineguns, but they are not as effective as you would expect. Their main use is for relocation soldiers or evacuation of infantry and cannons. The greatest upside of those vehicles is that they can haul cannons. The crew of the cannon loads on the vehicle automatically, regardless of whether, or not, there is room for them. You can also relocate cannons without their crews and deliver them into a new location. It is very important for many campaign missions. They are of invaluable help when you have to defend, or relocate often. By pressing E key, you unload the vehicle and selection will change immediately from the vehicle to the infantry group. This way, you can flank canons and attack fortified positions with infantry.
Repair and delivery vehicles are yet another group of technical support vehicles. They are highly vulnerable to enemy fire and a single shot, fired from any cannon, can destroy them. Try to keep them in the back. Use them after you have checked the area with infantry, to prevent any surprises. Repair vehicles can fix critical damage of a vehicle, i.e. a broken thread or damaged engine. If you have the skill Mechanics, from support doctrine active, you can restore the vehicle and its resistance up to a half of its original pool. Thanks to this, tanks and other armored vehicles can continue their service and you will avoid unwanted losses. Delivery trucks supply: ammo and fuel. This way, your vehicles and infantry can continue fighting. Trucks share a single meter for ammo for all the vehicles, so you do not have to worry about whether a given truck supplies ammo for infantry, mortar shells or AT shells. The heavier the caliber of ammo, the more it consumes the truck's supply points. The most powerful of tanks, or artillery, can quickly empty their delivery trucks and a single infantry squad will take long time before exhausting their supplies. You cannot restock special skills while in the battlefield (e.g. smoke grenades). Delivery and repair trucks can automatically get to their job whenever there is a need for that.