Because they are temperature-controlled and contain a refrigeration unit. In biological and medical applications, you might want to have the samples kept cool

Because they are temperature-controlled and contain a refrigeration unit. In biological and medical applications, you might want to have the samples kept cool for stability. You'll transfer them from a fridge or on ice into the cooled centrifuge and back again after you've spun it down. Since in these big units the rotor is often a big hunk of anodised aluminium, it also helps maintain temperature stability.