Light vehicles play a vital role in Blitzkrieg. The ability to move troops and artillery rapidly is essential in order to be able to exploit the gains made by armoured spearheads. If you can move faster than the enemy then they will struggle to keep up, let alone offer concerted resistence. The alternative is moving troops and guns by horse or by foot and neither have the speed or reliability to cope with modern warfare.
Motorcycles essentially replace mounted couriers and scouts. These bikes are equipped for a combat environment, with a side car mounting a machine gun for self-defence. As a scout the bike sports a huge vision range and the ability to sniff out concealed enemy troops and buildings. Since they have a high top speed and can uncover stealted enemy units they make great sniper-hunters.
Opel is a subsidiary of the American car firm "general motors". The Opel Blitz is a mass produced vehicle which provides the backbone to the Whermacht's logistics. The Blitz is used to rapidly move soliders as well as providing more mundane supply duties. A special version is availible at the Command Centre which carries cash. This allows for funds to be sent to an ally.
The Sd.Kfz. 251 is an APC and prime-mover for artillery. It's thin skin of armour gives it some protection against enemy small arms fire allowing for its troops inside to assault enemy positions from a more protected position. It also has a machinegun to support the troops as they advance. Its payload of sub-machine-gunners makes it very good for assisting assaults on enemy garrisons.
The Panzerwerfer is a multi-launch rocket system. It is simmilar to the Soviet Katyusha but superiour in several ways. Firstly its rocket launcher is much faster to aim and so can be launched without a setup time. Secondly it fires much larger and better designed rockets. These have a longer range, more power and are more accurate. However its high cost means it can never be used in massed batteries that the Soviets employ.
Bulldozers are essential for any large scale construction. Bulldozers are used to build all the buildings in the Axis roster and to keep them repaired and in good working order. While unarmed this role makes them the cornerstone of any war effort. Since they are only produced at the command centre, it is essential to keep spares on hand in case it is destroyed and needs to be rebuilt.