The Primarchs looked into him and identified him as Alatron.
Who is substantially less important than "what." Is he a god? Another Living Saint? Some weird warp thing? Is he related to the curses in some fashion? There's all sorts of ways to use a trustworthy warp entity, starting with summoning it to beat your enemies up. Arethea could teach our Twin Elite Alpha Demonologists a noncorrupt ritual if she were so inclined.
Also, his blood is probably a ludicrously powerful alkestral regent and one of the few things that I'd consider worth trying to enhance an Astartes with.