User blog comment:Supahbadmarine/By Tzeentch! Here We Go!/@comment-25033299-20130211071701/@comment-4608429-20130211211600

The Primarch where created by the Emperor, but the actual orriginal purpose of them is unknown. The Space Marines where created from the left-over genetics that could be salvaged from the Luna base, so where a sort of "Crap, my Primarch are gone. Oh well I'll see what I can do with what's left over. Oh this works well, I think I'll keep them around." Or something like that.