Because they're very space-saving, if you think about it, and I would expect that to come from the Japanese, who have a much smaller country than we do over here, in the United States, and they like very compact, neatly organized things. One would say they make the most efficient use of their space possible. So obviously if you want to have a giant, destructive robot, you might as well have it be your automobile, or your jet fighter, as well, hence the practicality of a transforming robot. So not only can you take your jet fighter to the grocery store, you can also transform and crush the grocery store with your foot.
"Autobots! Transform and Roll out!"
— Optimus Prime, Transformers
"Decepticons! Transform and Rise up!"
— Megatron, Transformers Animated