To answer the why, when the new version is released there are usually some corrections made after release and new ISO's posted so if you use cdr then you have to burn each one if you want it with cdrw you just re-write it saves disks.

If you use rw then you should just use the normal make disk from iso image option that you would normally use to burn a cdr.

If by making changes to the OS means you want to install additional software no you will have to install to HD.