Why delegation exists: apathy is a rational response, not a failure
Most token holders, even ones who care about a protocol's success, don't want to read every governance proposal, understand its technical tradeoffs, and cast an informed vote on a recurring basis. That's not laziness, it's a rational allocation of limited time and attention, especially for holders with a small stake where the effort clearly outweighs their individual influence on the outcome. Delegation exists as a direct mechanical answer to this: instead of forcing every holder to either vote uninformed or not vote at all, a holder can assign their voting power to another address (a delegate) who has chosen to specialize in following governance closely, while the holder keeps ownership and economic exposure to the underlying tokens.
Mechanically, delegation is usually implemented at the token contract level: a holder calls a delegate function specifying an address, and from that point forward the governor contract counts that holder's balance toward the delegate's voting power rather than the holder's own, until delegation is changed or revoked. Critically, delegation in most designs doesn't transfer the tokens themselves, only the voting weight attached to them, so a holder isn't giving up custody or the ability to sell, just the act of casting votes with that balance.