Trailer
A trailer is a cargo carrier used to transport heavy or large amounts of cargo from one point to another. Although such a purpose is useful in of itself to a Transformer, many other Transformers find ways to alter or modify them if their alternate mode is capable of pulling a trailer.
If you're the leader of the Autobots, it's pretty much a rule that you have to have a trailer. There are a few exceptions.
Fiction
Generation 1
Marvel Comics
After being rebuilt as a Powermaster, Optimus Prime was able to combine with his trailer to form a super mode for himself.
Generation 1 cartoon
Optimus Prime stored Roller and his combat deck in his trailer, using them whenever the need arose. This included the battle to prevent the Decepticons from leaving Earth and returning to Cybertron. However, during the battle, Megatron destroyed the trailer. Not to be out done, Optimus Prime merely "pointed" at the Decepticon leader, causing him to somehow receive heavy fire. More Than Meets the Eye, Part 1
Over the course of the war, Optimus Prime would use the trailer to contain extinct herbivores Dinobot Island, Part 2, transport Elita One to Alpha Trion's lab The Search for Alpha Trion and carry the Dinobots to Sub-Atlantica, even though Grimlock has at least a few feet on Prime Atlantis, Arise!. If anything, the trailer is roomy.
Robots in Disguise
Optimus Prime is capable of combining with his trailer to form a super mode, while Ultra Magnus combines with his trailer to form his body.
Unicron Trilogy
Armada
Optimus Prime is capable of combining with his trailer to form a super mode, making him taller than Megatron at least.
Energon
Optimus Prime's trailer contains his drone units, which combine with the Autobot leader to form his super mode. The trailer itself is nothing special.
Cybertron
Optimus Prime used his trailer to combine with his body to form a super mode. Oddly enough, in vehicle mode, Prime's trailer held rather large cannons, despite the fact that it was a fire truck. Scattorshot brought this up, but Prime merely replied that a fire truck was larger than a tank. Inferno
Animated
Optimus Prime's alternate mode is technically a fire engine, meaning that he shouldn't have a trailer. However, Prime is capable of detaching from the engine into a smaller truck which is capable of pulling other trailers. Transform and Roll Out! Headmaster Garbage In, Garbage Out
Trivia
When Optimus Prime transforms into robot mode, his trailer usually disappears, only to reattach itself to Prime whenever he goes into vehicle mode. Many fans theories about this disappearance have arisen over the years, with the most popular being that Prime stores the trailer in subspace.
