why do people suggest burning live-cd's instead of live-usb's ?

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http://www.linuxquestions.org – i've come across some responses to posts lately that recommend burning a live-cd or dvd of something in order to troubleshoot an issue. now that cd's and dvd's are pretty much now obsolete technology (the last two pc's i bought didnt contain a dvd drive) why do people prefer live-cd's/dvd's rather than live-usb's ? (HowTos)