Because online games have latency and jitter, if your client isn't aware of a player before it walks around a corner you'll get a very annoying pop-in effect as

Because online games have latency and jitter, if your client isn't aware of a player before it walks around a corner you'll get a very annoying pop-in effect as the player "appears" small distance from the corner. I believe SCUM is a game that have this. This kind of culling is also quite resource intensive on the server for games that have many players, large maps or complex geometry. CSGO has partly implemented this, your client only gets information through some walls and objects, so you cannot see a player on the other side of the map. But it has quite simple maps and is overall not a complex game.