It's a thermodynamics thing. Reversible processes are the most efficient (something to do with entropy). Deleting information means it's no longer reversible. This is an entirely theoretical thing. There are theoretical limits to energy usage of computation based on this, but actual computers are nowhere near these theoretical limits, at all.
Edit: and yes, most of the logical operations in a regular chip like AND, OR, NAND etc are irreversible (in isolation, anyway)
Edit: and yes, most of the logical operations in a regular chip like AND, OR, NAND etc are irreversible (in isolation, anyway)