When the payee deposits the cheque into their account, the cheque is verified as genuine (or 'cleared', a process typically taking several days) and the transfer is performed (usually via a clearing house or similar system).
By contrast, when an individual requests a banker's draft the amount of the draft (plus any applicable fees and charges) is immediately removed from their account.
(An individual without an account at the issuing bank may request a banker's draft and pay for it in cash, subject to applicable anti-money laundering law and the bank's issuing policies.)
Because the funds of a banker's draft have already been debited they are proven to be available.
Like other types of cheques, a draft must still be cleared and so it will take several days for the funds to become available in the payee's account.