Some people prefer to know exactly what their computer does, either to enable debugging in obscure cases or due to security concerns.

Thanks to Intel, who has invented the completely unnecessary System Management Mode, to compensate for the laziness of Microsoft, who could not be bothered to update MS-DOS and Windows with some features required in modern computers, now the BIOS has the ability to do whatever it wants on your computer, without this being detectable by the owner.

Hopefully the BIOS writers do not abuse this, and the many problems caused by BIOSes are due to unintentional bugs and not due to malice, but it would still better to be certain that your firmware does not do anything nefarious.

When debugging hardware problems, it is also much simpler when you are certain about what the computer really does, instead of not knowing whether the BIOS takes control when certain hardware events happen, overriding any policies that you may try to implement in your operating system.

Replacing the proprietary BIOS still does not provide complete control over what you own, as there are auxiliary CPUs with their own agenda, but it is still much closer to full control than when you do not know what the BIOS does.