The metering system goes to sleep to save power. Usually after somewhere around 3-5 seconds.
E.g. if you half-press the shutter then you will "wake up" the metering system and it will go active and stay active (even if you let go) and as you point the camera around you'll see the suggested exposure changing (unless you press the AE Lock button... in which case it will lock in the current metering and it will remain locked until either (a) you take the shot, (b) the metering system goes to sleep (after 3-5 seconds), or (c) you press the AF selection button (which is way to force cancel AE lock without waiting for it to time-out or take a shot.)
Back-button focus is useful when want to control *when* the camera focuses. It's especially useful for sports shooters who are tracking an athlete, but have to deal with things that get in the way from time to time (another player that you aren't in (e.g. they don't have the ball), a spectator, etc.) this way you can set focus when you *can* see the athlete and it'll remain focused to that distance... if someone gets in the way it will not refocus (unless you press the AF button again.) If nobody is in the way then you can press-and-hold the AF button (if in AI Servo mode) and the camera will continuously adjust focus as the subject distance changes.