Well, partly vibration transfer, and partly physical stability. With any nut you get a fair bit of forces acting upon it when playing - tuning, string bends and maybe trem operation. Though it's not in a slot, a Gibson style nut is a lot wider, so is a lot more stable compared to gluing a thin Fender-style nut in the same position. So a Fender-style nut needs some support both front and back in order to keep it nice and upright and resist moving under string influence. With a gap at the back of the slot, the forces on the nut will want to pull the top edge forwards, so the bottom of the nut is pivoted backwards. It then won't be too long before the glue holding the nut breaks, and the nut comes lose. A tight slot resists the twisting motion, so your nut stays nice and firm for years.