Without it, you only get unaccelerated frame buffer graphics.
This is because the X.org / XFree86 drivers for Intel and ATI, as well as the Linux text console, rely on the Video BIOS and Apple's firmware only provides a Video BIOS when booting in BIOS compatibility mode. In addition, if you want good graphics support in Linux, you must boot it using the BIOS compatibility mode. Recent firmware adds a BIOS compatibility mode, including detection of BIOS-bootable disks and CDs in the built-in boot volume chooser. This is important - some older versions of firmware won't provide some of the underlying support that you'll likely need to install Debian. Verify that your computer has the latest firmware. macmini2,1 installation specials and pain.
