As far as I know the more you increase your destruction skill
- the lower is the cost in magika of your destructive spells
(actually for my character (91in destruction without considering bonuses ) spells cost 1/4 than former)
- the higher is the degree of efficiency of your destructive spells
(actually the efficiency of my spells is of the 95%)
morever (and this is IMO the most useful improvement), if you achieve the necessary lv you can create tremendously powerful spells !!
I created f.i. the "EXPIATION TOUCH": 50pts Fire Damage + 50pts Frost Damage + 50pts Shock Damage on touch and it does cost just about 100pts of magika !!
Awsome! no?

It is extremely useful as you can use it even against atronachs which, as you may know, absorb the damage of their elemental sphere.
I believe that any skill in excess of 100 raises anyhow your abilities... indeed i reached 100 + 35 in blade and the damage caused by swords changes in proportion to the equipments and the relative bonuses. (i think the same goes for destruction.)