Timothy Murphy:
Does anybody nowadays actually burn CDs or DVDs?
Ed Greshko:
Yes..... The BIOS on many of the machines I encounter don't support booting from USB.
Or the BIOS is none-too-great at booting from USB.
Just recently I had to boot from USB to install onto a laptop with no optical drive. I found four different methods of putting a Linux installer onto a USB. One of them involved Windows, so that was out. Two of them were Linux programs for writing installers to the flash drive. One required adding things to a current install that I didn't want to do. The other was less painful, and worked for writing one installer to a USB, but not for another (I tried Fedora and Ubuntu on this laptop). The last was plain dd if=/install.iso of=/def/flashdrive (I'm paraphrasing), and that worked for the one that didn't work the other way.
All of which was more hassle than burning a disc, and was actually slower than burning a disc for one of the methods (I don't recall which, but I tend to think it was the dd method).
So, while you can install from a USB, it's not always the most convenient.