I've only watched a couple of episodes of it. It can start of kind of interesting with a look at various ancient cultures with odd things going on, like one I saw where they were talking about some Mayan/Aztec/whatever city which was organized in a pattern which remebled a computer circuit. Then out of nowhere (admittedly, I was interrupted and pulled away from the show, so I obviously missed something) they have CGI graphics detailing alien ships landing at this city, and how the layout of the city was meant specifically for landing and take-off for alien ships. Althugh I did find it interesting to learn in another episode that the Nazis really did have their own official organization for investigating the paranormal.
As for why History airs it, well, at least it does have some sort of historical content. Moreso than shows like Pawn Stars, American Restoration, or Swamp People. And I say this as someone who likes Pawn Stars.