The firmware included with the Raspberry Pi 2 Model B version 1.2 introduced the possibility of booting from a USB device: before that, it was only possible to boot from an SD or microSD card. But ...
It used to be typical for every variation of Raspberry Pi to require booting from an SD or microSD card. It wasn't until the Raspberry Pi 2 Model B that the firmware received an update for booting ...
For such an inexpensive, tiny little SBC, it certainly gives me a lot of fun and assistance in a number of projects ...