The reason that the client may need to know about other players locations when they are not visible is sound.

You can hear somebody moving on the other side of the wall to you and for the sound to be realistic it needs to be played at the correct volume level determined by distance etc.

This isn't a big issue in some games , because you can just make peoples movements silent (of course there is still weapon sounds). However other games (like CS) have a stealth element where you need to make a tradeoff between moving quickly and noisily or slowly and silently so the sound is a big part of that.