Many people take privacy for granted when it comes to the network world of computers.
The two types of passwords give you different capabilities to alter the BIOS configuration. If a supervisor password is also set, then only the supervisor has the capability of changing any of the options. The user password can change just a subset of the BIOS options. When you turn on the computer and a BIOS password is set, the boot sequence processes the information in the BIOS and then stops to ask for the password. You have three tries to get the password correct.