Could someone explain why it is beneficial or necessary to ensure that equipment is level? I can understand this with turntables but I can't understand how it could effect CD player, DAC, pre or power amps.

I'm using dissimilar support devices under each component, unequal in height by 2-3mm.

Many equipment instructions insist that the unit must be levelled. I can find a way to address 2-3mm if really necessary and I've done it but can't hear a difference.