Seen a lot of boats usually 27+ ft. and cruiser type for sale and
there's lots of mention that the craft is "Coast Guard Certified" and
has some certification number. I've seen this mostly on salt water
craft, but also river and even lake boats.

What does it mean to have a CG certification? What is the benefit of
the Cert.? what case's is the cert. mamdatory?

ie what is it? and what does it do?