Windows 8.1: UEFI or BIOS?

Hi,

What's the big deal with new UEFI? My new computer has UEFI but when I need to boot from an (older) external drive, I need to put it into BIOS.
I probably need to make a specific usb stick to install Windows 8.1 via UEFI but what 't my added value?

If I boot only from VHD's, does it matter and can I make my system boot automaticaly whatever BIOS or UEFI is enabled?

Please advise.
J.

Jan Hoedt