I have no direct knowledge, but my best guess is that they have significant legit customers using these as an alternative to Microsoft's own Key Management Services. Historically, some stuff like this has even turned out to be used by the company themselves (e.g. a few Steam/GOG releases have been found to use old warez scene cracks, official arcade emulators using bootleg sets from MAME, etc.).

I use my MSDN keys generated back in 2015 over and over. It works great 9 years later.