It's really just about making hiring easier. If you can say that someone with a Master's in CS will know all these things we think are important, then you can set the bar at MSCS and ignore all the people with a BS even if they have the same knowledge.

Having the MS means you filter out a bunch of people that may or may not have the skill you need. The filter reduces the number of resumes you need to evaluate.

It's stupid, but makes sense in the eyes of HR and as long as you can fill your pipeline with enough people, it works.