You don't have to wait for the camera to focus or wonder if it focused, because it's already done. It will make snow gray and black rocks gray. Sometimes you don't want things to be gray. These are instances when you would use the exposure-lock feature. In order to use the exposure-lock feature aim the focus point at a medium gray object or scene.
Green leaves look gray to the camera's meter.
Asphalt pavement is close to medium gray as is some tree bark. Try to see things in black and white and average out the values in your head. In these two pictures, the camera was pre-focused, activating the exposure lock, on the areas indicated. When the exposure is locked on a lighter area the whole picture is darker. You can pre-focus and reframe any way you want.