When a recording engineer produces a stereo mix from the various tracks, he is attempting to simulate a performance coming from a stage. The sound reaching the ears of a listener in the audience does not come from the side, so the studio monitors are placed out in front of the engineer while he blends the tracks between the channels to simulate the way the sounds reach the ears at different times. In order to reproduce the sound the way it was mixed, it is necessary to set the speakers out in front like the studio monitors.
Placing speakers this way is second nature to us audiophiles, so it always shocks me when I see someone's speakers set up differently, for example, aimed at each other from opposite corners of the room.