I really have no idea who hard games are for. Back in the 90s, sure, hard games gave you a bit more value out of a game which could probably be beaten in one sitting if it wasn't so hard. That would suck, since we only got new games for our birthday and Christmas. Also useful for developers, since making the player replay the same part over and over, meant you needed less assets and could use smaller, cheaper EPROMs.
But why are they still around today? We have more games than we could ever hope to finish thanks to Steam. There's no need to artificially extend the runtime of a game with difficulty, you can just start a new game if you finish it. So who are they for? The only people I can think of, are people who are looking to have some kind of accomplishment to make up for the fact they feel like they don't have anything like this in their real lives.