Substantial difference in Player's skill level or rank
In multiplayer games, when the game won't distinguish between pro player and casual player, it really makes the game frustrating for the casual player. It's like saying to the younger school kid, who is playing to learn the football game-play, to play against the pro football player, In addition to that asking him to play against the pro football player every time. It will definitely result in frustration for kid (new player) and leads him to stop playing football forever. Now put this example, in multiplayer arena games, if the game won't introduce any rank system or distinguishing system between the players of different skills, it will negatively effect the player's gameplay experience. This problem is present in many new emerging multiplayer games and the players stop playing it because of these reasons. For Example: Ark Survival of the Fittest, even top rank players in the whole world will be matched against a new novice player with no prior experience. Same with the game Heroes and Generals.