This comes in very handy if there are any irrops (ATC, weather etc.). I have been on several CO flights in the past where there are minimal schedule padding and a 20 minute delay in departure usually results in a 10-15 minute delay in arrival (not much, but when your departing gate is at the other end of IAH, it's a difference between walking and sprinting).

Plus, IMO general fliers usually appreciates that "We're early" announcements instead of "We're late 5 minutes" and they don't know how much UA pads its schedule.