I notice that many but not all of the sgians dubh have a row of serrations on the back of the blade.

What are the historical basis of this?
the reason for it?

I do remember a story from "All Quiet on the western Front" where an old sargeant was warning the recruits that a "pioneers'" blade would get them executed out of hand if captured as it was felt that the "sawback" bayonet they used was dirty pool