Back in the old days we would do backups using “dd” of the entire disk. Therefore, we had the “c” slice so that we could do it all with one command.
That’s why the “c” slice exists.
DD isn’t perfect. If a disk is only 10% full you spend 90% of your time copying blocks that are “junk” or (for example) are used for “swap” (useless to be backed up). “dd” is a waste of time unless your disk is nearly full or if, for some reason, you need an exact, block-by-block, copy.
This was all before RAID-0 disk mirroring and volume managers did all that kind of partition copying for you.
(Someone mentioned “dump” on the “c” slice. That won’t work. “dump” is a file-by-file copy [actually, inode by inode] so that won’t work.)
Someone else asked “why is it c, not the first partition, or the last”. The answer is “tradition”. I can only guess that Ken or Dennis (or possibly Bill Joy or Kirk McKusick) had a good reason at the time. I assume they had used the first two partition labels for actual partitions. Then one day someone had the idea of the over-lapping partition for doing backups and “c” was the next partition available. Since there were only 2-3 Unix machines at the time, doing this twice can “set the standard” that gets used for the rest of time.
Another example of how historic accidents become standards that never go way is described in this article: Understanding the bin, sbin, usr/bin , usr/sbin split