Why should I trim leaves off of my squash plants?

Why should I trim leaves off of my squash plants? I want the squash to grow as big as possible. I have seen pro gardeners snip off certain leaves of a vegetable plant. Any other tips on how to grow my squash plants would be very helpful. It is August 11th and my area has had 75/100 days of rain this summer. It worries me that the plants aren't on schedule. The vegetables have just started to grow neath the flower this week.