I was taught that the reason for using matt has three purposes.
first...If you are laying up a laminate over gel coat working from the outside/in...the matt (thin angelhair) helps prevent the texture of the woven fiberglass
from showing through the gelcoat
the second was to build up thickness of a laminate quickly and cheaply.
the third was that a final layer of matt over the laminate of a deck
for example, helped create a barrier against water
intrusion into the woven layers.