Porteus

Hiyall,
I'm trying to get my head around this issue. What is the reason for using Porteus on a USB?

Portability. You can take it with you when you visit relatives, friends, etc. A backup for your system if the hard drive fails to boot.

Added in 11 hours 54 minutes 27 seconds:
An easy and safe way to test a new release or OS without risking the harddrive.

Also, thumb drives are often faster than hard drives. It's an easy way to make a slow computer usable again. Or maybe you want to do something like me: I will remove the Hard Drive from an old PC to use it as backup and I'll be installing linux on an 8 or 16GB USB drive.