In addition to making the toys cooler than anything else, ever, there is a role of transformation in fiction. This is not as straightforward as it sounds. Since people relate best to humanoid figures, and since a Transformer in robot mode can usually do anything necessary for the story, the creators need to provide reasons for them to transform to their altmodes. Here is a sampling of those reasons.
The original reason was that they were "robots in disguise". In Generation One, upon arriving on Earth, the Transformers took on Earthen altmodes so as to move unobserved among human society. This is also the primary reason for Transformation in the current IDW continuity. In most continuities, these disguises fool both "the locals" and other Transformers, though there are exceptions.
While usually the disguise justification only applies to A) Transformers with mechanical altmodes, who B) take on Earth-based forms, there are variations. In Spotlight: Ultra Magnus, the titular hero takes on the form of a Nebulan truck so as to remain disguised on that world. In some continuities, the Pretenders can use their shells to disguise themselves as normal organic beings. (In other cases, they're just disguised as "not robots".) And, in a few instances, Transformers from the Beast Era use their ability to mimic normal animals to hide undetected.
In those stories set before the Transformers come to Earth, altmodes are usually not disguises. The primary justification for them is that they are better suited for getting from point A to point B quickly.
Transformers sometimes have abilities that cannot be accessed in robot mode, which may include:
- Construction capability (e.g., a crane)
- Improved hauling capacity
- Certain of Soundwave and Blaster's communication/audio abilities appear to be either unavailable in robot mode or are improved in tapedeck mode.
- Looking at very small things
In the first season of Beast Wars, the Earth's vast energon deposits were actively harmful to Transformers in robot mode. The synth-flesh of their adopted beast modes, however, could protect them from the radiation. They thus used their robot modes outside their respective bases only when necessary.
During the conflict chronicled in Beast Machines, the Maximals' technorganic bodies were invisible to Vehicon sensors, but only in beast mode. The Maximals would therefore fight in robot mode, but transform back to beast mode when they needed to sneak around.
Altmodes can prove to be more efficient in certain combat situations than robot modes. For example:
- A Seeker in jet mode would be more maneuverable than a non-aerodynamic flying humanoid robot.
- A tank altmode would provide improved physical protection while also allowing for a more stable firing platform for the main weapon (they design tanks that way for a reason)
- A beast mode may be more physically overpowering in close combat and simply more intimidating
Weapon systems integral to the Transformer are often implied as being more powerful in altmode. When Megatron really wanted to get down to business, he would transform and hand himself off to a fellow Decepticon. The same reasoning could reasonably be applied to Shockwave and Galvatron.
During the "Unicron Trilogy", Mini-cons interacting with their respective bulks transform into their alternate modes before linking up. In addition to that, combiner teams of Mini-cons would transform into a section of the combined mode. Combining teams are also made of 5 or 6 robots, who all transform into the respective part of the combined super robot. Headmasters and Powermasters all need to transform into a head or engine before linking up to their partner, who is unable to transform when he is not linked with his or her partner.