All sorts. Having a full bootable OS on a CD or USB was always cool. When I left the military and was a security I used to use them to boot computers in the buildings I worked in so I could browse the internet.

Before encryption by default, get files from windows for family when they messed up their computers. Or change the passwords.

Before browser profiles and containers I used them in VMs for different things like banning, shopping, etc.

Down to your imagination really.

Not too mention just to play around with them too.