Two notes about this: 1) It's not that common any more except in older buildings. 2) Historically, this was because only cold water was deemed potable because

Two notes about this: 1) It's not that common any more except in older buildings. 2) Historically, this was because only cold water was deemed potable because it came directly from the mains supply, whereas hot water would be lying around heating tanks etc. and might be contaminated. So by keeping them separate, you always knew the cold water tap was safe to drink from.