In-ceiling speakers and In-wall speakers are used to reproduce sound in several situations.
House wide audio: Usually in-ceiling speakers are recommended to broadcast a wide spectrum of sound to fill the entire room.
In-wall speakers are used primarily for directional sound typically around ear level in height.
Theater room: In-wall speakers with multiple-drivers (3-way or 4-way) are placed around ear level in the front, sides, and rear walls of the room. Sometimes, we use rear in-ceilings (with aiming tweeters) if the rear row of chairs are against the back wall.
The most important thing to remember when choosing a loudspeaker...
"Good speakers aren't cheap and cheap speakers aren't good"
Doug Van Arsdale
Custom Home Theater