Trashing is good because it ensures the drawing of action cards more consistently. When trashing weaker cards, a smaller deck size is kept, ensuring more reshuffles faster, more draws of those same powerful cards to be replayed, and more consistent hands. In short, trashing the starting Coppers and Estates as quickly as possible is reducing the amount of clutter in a deck.
An example of this is the Chapel strategy. By purchasing increasing valuable treasure and removing less valuable treasure/victory point cards, you ensure that the majority of your draws will be Silver or Gold, thus increasing your likelihood of getting to 8 treasure and being able to purchase a Province.