Fictional Friday: Power Word Pointless

Why are the spells Power Word Stun and Power Word Kill Enchantment spells?

Why are Enchantment spells linked to hit points?

Does anyone even use the spells any more?  I mean, seriously, starting with 3e, and continuing into 4e and even worse with 5e, what CR20 monster has less than 100 hit points? And if your party has managed to smash it down to under 100 hit points anyway, why waste a 9th-level spell on it? And how are you supposed to know when it's under 100 hit points, anyway?

I mean the CR 21 dragon has 267 hit points, the CR 20 pit fiend has 300, the CR 15 purple worm has 167 and the frickin' CR 5 hill giant has 101.

"Hey, you know, now that the fighter has hit the stock, classless, non-advanced, henchman hill giant with the javalin, I'll pop off my 9th-level instant death spell, because now I know it won't be wasted."

Bleah.

And Power Word Heal is just a really tightly focused, and super-convenient, video game-inspired wish.

Has anyone ever really used the Power Word spells, or do these just keep getting dragged forward as one of those legacy things that just has to be there, or purists will bitch up a storm?