For those of you who want to know how to boot from a USB Flash Drive, BootDisk.com has a great tutorial on that, featuring 6 different methods. – link
Btw, most computers these days support USB boot, you just need to make your USB flash drive bootable by copying boot images into it.
When do you need to boot off your USB drive? Well, on netbooks and any of these new ultra-thin and light laptops (such as my ASUS UL30Vt) that doesn’t include an optical drive. Frankly, I just think DVD/Blu-ray only adds more weight to your laptop, it’s overrated. Just get an external drive if you really need it and also you can use virtual CD/DVDs for carrying things on the go.