Answer:
BIOS is one of the first things that starts when a computer is "turned on"
This means that if you update is aborted, and the BIOS is not installed correctly/does not work correctly, it may affect how your computer boots when it starts to run, and things like the operative system may never turn on.
In other cases the BIOS may start correctly, but it may have some problems that lead to an unnestable state of the computer.
In modern computers, the previous BIOS is stored in the memory, that may solve problems like this for cases in where the power goes of in the middle of an update or things like that.