The PBA prevents anything being read from the hard disk such as the operating system until the user has confirmed they have the correct password or other credentials including multi-factor authentication.
[2] The PBA prevents any operating system from loading until the user has confirmed he/she has the correct password to unlock the computer.
[2] That trusted layer eliminates the possibility that one of the millions of lines of OS code can compromise the privacy of personal or company data.
For mobile devices that can be stolen and attackers gain permanent physical access (paragraph Attacker with skill and lengthy physical access) Microsoft advise the use of pre-boot authentication and to disable standby power management.
Without cryptographic protection of a hardware (TPM) supported secure boot environment, PBA is easily defeated with Evil Maid style of attacks.