I fix a lot of computers too Josh, and you are right for the most part you don't need a tool like this bootable OS on a CD, but on those systems that have crashed hard, and will not boot at all, I have been able to back up peoples photo's, music, .doc files, email, etc as I can burn to CD or DVD. Knoppix has these tools. I can also dump data to my USB thumb drive.
At the end of the day all people care about is having their data intact.
Once I have the backup done, I can format the drive if need be.
Also, and we have all seen systems with files that just don't want to be removed, even Killbox can't remove some things, but boot into Knoppix and I can take care of it.
And when I have to remove stubborn files, this method does it so quickly. And only one re-boot needed.
It's a time saver.