In general, the strongest focusing point in in the center of the frame. But sometimes you don't want the focused subject to be in the center. Or maybe the subject doesn't have enough contrast to focus on. So what some photographers do is they look through the viewfinder and focus on whatever they want to be in focus. With the focus locked, they then move the camera, recompose the shot and take the picture. This also applies to metering as well. Sometimes subject of the photo is lighter or darker than the rest of the scene. You can meter off of something other than the subject, lock the exposure reading, recompose and take a picture with different exposure settings than it would have been had you just taken the picture.