USB drives generally aren’t bootable by default, however you can make them bootable with third party tools available on the internet, with bootable flash drive you can carry Windows PE bootable OS on it to troubleshoot and fix Windows booting issues, further more these days Windows and non-Windows OSes can be installed from pen drives, (your BIOS needs to offer the option to boot from USB drive) MakeItBoot is freeware portable app which makes your USB stick bootable.
Making Pen Drives with MakeItBoot
Download the program zip file and extract its contents to a folder and runt the program executable. Inserts the flash drive, click “Refresh” button on program window and select the USB drive and click Start button.
MakeItBoot works on Windows XP/Vista/7.