The troop transports is a fast, but lightly armored infantry transport. It can repair other vehicles.
This lightly armored motor vehicle has several important uses on the battlefield. While mainly used to transport infantry to the front, it can also perform emergency repairs on any vehicle on the battlefield. Furthermore, its fast movement speed and long view range allows it to serve as a recon unit and perform quick raids on enemy positions. Its machine gun is mostly useful against infantry and light vehicles, and its armor isn’t strong enough to withstand more than a few hits.
Despite having a machine gun, the troop transport (or jeep) is all but worthless in combat due to its extreme fragility. Rather, the jeep's main purpose is usually to provide a repair station for friendly helicopters in a safe rear area (jeeps can repair any vehicle, but ground units aren't as mobile to be able to retreat to the rear). As "repair stations" it's best to keep your jeeps stationary or at least move them only over short distances so that air player teammates don't have to search for where you've parked them or chase you around for repairs (alternatively being stationary makes you vulnerable to being destroyed by cheap tactical aids like tank busters).
Ideally, an infantry player should probably use some of his reinforcement points on jeeps for repair stations, though this often doesn't happen. This means that an air player will either have to call in a jeep with his own reinforcement points (rather expensive at 800), or expend tactical aid for one or more airdropped jeeps. The advantage of spawning a jeep with reinforcement points is that the jeep is easily and painlessly replaceable should it be destroyed. Calling in a jeep as an air player at the beginning of the match is also useful as it allows you to participate in capturing a few command points and earning some TA. The downside of course, is that you have fewer reinforcement points to spend on helicopters to actually fight the enemy. On the other hand, using an airdropped jeep frees up reinforcement points and allows you to build up to your full complement of choppers, but as the airdropped jeep costs TA you should keep it safe and hidden to avoid its destruction.
Of course jeeps can also be used to transport infantry, but since the jeep is pretty useless in a fight and is about as durable as a cheaper transport truck, the latter sees more use instead.