The reasons to leave a gap between cage and pad are to allow air to circulate and to keep direct pressure off of the pad itself. This prevents the pad from shorting out and/or causing a fire. I had a pad short out on me because there was too much weight on it some time back, but fortunately there was no fire. It is also helpful to be able to somewhat regulate your temps that way, by increasing or decreasing the gap, but ideally that would be done with a thermostat instead.