“The defining feature of procedurally generated games is that experiences differ.” (Green, 2016)
Procedural generation can bring a sense of the unknown to a game. In a traditional hand crafted level every player will have largely the same experience, items will be located in the exact same locations, and the environment & layout will be identical. Playing a game where the level layout is predictable can lead to stagnation and hurt the game's replayability. If the player knows the narrative of the story ahead of time and where necessary progression items are located they may not be as motivated to keep playing, as the sense of discovery is lost.
However, if your game employs procedural generation, then the challenge can be fresh each time the game is run. The game is always evolving; the environments are always new.
A good example of this is the generation of the world map in Sid Meier’s Civilization, in which the fog of war obscures the land waiting to be discovered. Through the use of procedural generation the player’s ability to predict where the land is placed is reduced.
“TAKEAWAY: Procedural generation can produce a new map every time players log in to ensure that they'll explore and experiment, though you'll get better multiplayer balance without it if discovery isn't a core gameplay loop.” (Moss, 2016)