Why do snakes come out of their burrows during summer?

Because they are warm and need to feed, amongst other reasons.
Remember as they as cold blooded reptiles they behaviour differently in the warmer months..

I know this is an older post, but I adore snakes and want to answer.

Ben is right, being warm blooded makes the world of difference. The few snakes I have caught in mid and late fall were defenseless. The colder temperature pretty much made them immobile. They need that heat for energy... movement, digestion... without warmth they can't do anything. As mammals obviously we produce our own heat therefor not requiring it from our environment.