Canisters also had ventilation panels.
The technology arose in the days of the Old Empire, and was used to house the minds of great philosophers, called Cogitors. These brains were placed in the canisters and kept almost exclusively in a state of sensory deprivation, to avoid distractions that could taint the evolution of their thoughts. On rare occasions specially trained disciples would act as intermediaries between the Cogitors and the outside world.
Preservation canisters were later adopted by the Titans, as a means of prolonging their lives and thus their grip on power. After the rise of the thinking machines, Omnius also used the canisters to reward loyal humans by turning them into Cymeks. In both cases, the preservation canisters were fitted with sensory nodes, which allowed the brain within to directly interact with a robotic body.