I assume normal is default difficulty setting that the game designers had in mind for the game and designed normal to be not too difficult but still challenge.
So I recon
Easy: if you?re new to that gaming genre or finding normal too tough
Normal: the way the game is meant to be played
Hard: for the challenge of beating

the worst the designers can throw at you
Personally I prefer when games have one overall difficulty setting like in Zelda or GTA id rather have a bigger game than replay a smaller game with more annoying baddies
Though sometimes this can fuck up when a game AI is way too easy
e.g.
Black and White 2
Assault a god with 25 guys and 1 catapult