Being able to boot a computer from USB or DVD is very useful.  If you are making a USB get RUFUS from here:

https://rufus.akeo.ie/

Next do a little research on what you want to make:

https://www.lifewire.com/free-bootable-antivirus-tools-2625785

http://www.makeuseof.com/tag/the-ultimate-pc-repair-toolkit-in-your-pocket-ultimate-boot-cd-on-a-usb-stick/

http://lifehacker.com/5984707/five-best-system-rescue-discs

https://techtalk.gfi.com/top-5-free-rescue-discs-for-your-sys-admin-toolkit/