Also in harder difficulties is where the games systems shine at , mostly (I think of Bioshock Infinite as an example)
Yep, the number one reason. A lot of the time, reviews that say a combat system is too simple or repetitive just didn't have a reason to really get in there. I almost feel like they're selling the game short.
One, you really have to dig into the mechanics of the game. If I start on hard, I can kinda blast through it. But on super hard, I have to think and strategize and take advantage of things I didn't even know existed. Most apparent in character action games.
Two, the thrill of accomplishment. Everywhere in life, I look for challenges because I love to overcome them. Finishing a section in The Last of Us may be draining, but making it through felt great.
Three, longetivity. Instead of flying through some five hour campaign, it'll inevitably add some hours to your playtime. I prefer short games as I'm older but this was always helpful when I had less money.
Four, game's are really the only things that challenge me anymore after grad school (aside from guitar stuff). It makes me feel alive!!