Working on calibrated equipment is about having a neutral, uniform reference so that you can actually see what you're doing without having to second guess yourself. You can draw an analogy to reference monitors for sound mixing. Monitors have two properties: they reveal detail in the sound and they have a flat frequency response. The former is obviously important, and it's why as a designer you should have a nice monitor. The latter is important so that when you work in different locations, you don't have to recalibrate your expectations. There may also be non-linear perceptual illusions associated with certain aspects of the signal being attenuated or boosted, which would be another reason to have a calibrated/flat reference, but I'm not certain if this would be relevant at the scale we're talking about (between calibrated and the whole space of non-calibrated color profiles).