The reason he rolls the dice is:
To see if a monster or his armor can withstand an attack.
To see how much damage a monster does to a character.(characters get a roll for defence on this)
To see if a monster dodges an attack.
To see if the monster ressists a spell.
To see if the monster rolls a crit(see above "when monster attacks").
To see if something hears a character sneaking(ussually thief/assassin).
To see if a particullarly rocky cliff gives way and causes a climbing character to fall.
There is a cornacopia of things a GM could roll for, they inculde and are not limited to the things above.
Note: The overall reason the GM rolls a dice is because he has to pit the result against the character's roll(with augments) to see exactly what happens. If the GM rolls higher(I believe he rolls a different die according to the monster, such as you wouldn't roll a 100 side die for a large dragon to dodge a large attack since the dragon would not be fast enough to get that good an odds of dodging) what ever the GM was rolling for succeeds and if the GM rolls lower, the character wins doesn't not suffer bad effects from whatever the GM was rolling for.