Good to have if you work on people's machines. Not all computers support USB boot. But yeah I don't keep much CD's around anymore. Personally I got out of working on people's machines as it's not worth the effort for the little that people want to pay, so I'm kinda behind in that scene.
Ones I do have off top of my head:
-Hiren's Boot CD
-Ultimate Boot CD
-System Rescue Suite, or something like that... someone gave me it once, I used it a few times, I can't remember what was special about it but it had some unique stuff. Diagnostics and stuff like that.
-Pretty much any Linux distro live CD/USB. Great if all you want to do is grab all the data before you do a format. Ex: WIndows 8 machines. I find it's easier to boot a Linux OS and use GUI or command line to access files than trying to navigate the horrible UI of Windows 8.
I have an external hard drive that you can load ISOs on and then emulate a cdrom drive, so that's usually what I use. It's nice as you don't need to burn any CDs. Heck I don't even have a cdrom drive in my main machine. Though sometimes I think of putting one as it can still come in handy. But 99% of the time I'd be using it to do people favours like ripping DVD/BluRays so it's easier when I can just tell them I don't have one.