You can think of the flash drive like a bootable CD/DVD-ROM, if you wanted to restore your system backup of your c:\ (which would be saved on your 2TB external) you can’t boot your computer from the C:\ and perform the restore because it is in use.
Booting the the Macrium USB drive will boot a temporary operating system that is capable of reading the stored image from your 2TB external and have the ability to restore the data to your computers internal disk c:\
Let us know if you have any other questions.