To be honest, I have not backed up images on CDs for over 10 years now. But back in the days when a D70 RAW file was about 5MB it made sense. You could fit about 140 RAW photos on one CD. Or if you back up a few years earlier, photo labs would provide film scans on CDs, with the average size of 3MB per scan, a CD would hold over 200 photos.
Any why not? CDs were cheap, easily stored and cataloged and they would last forever. Right? Wrong. It turns out that the lifespan of CDs is not as long as everyone initially thought. And it also turns out that CDs are more susceptible to the elements than we thoughts. And your old photo collection stored on CDs and DVDs is probably dying s you read.