Home Grown Yeast

[Godstilla]

Hello All!

In J.W. Walstad's book "Simple Sour Mash to Simple Alcohol Fuel" it says: You can culture your own yeast as follows:

1. Mix one Pint of water, one Tablespoon sugar, and 1/4 cup flour together in a blender or hand mix thoroughly and boil
for 20 to 30 minutes.

2. After mixture has cooled to about 85 degrees F., mix in 4 tablespoons of Distillers Yeast.

3. Place in a quart jar with a lid and punch a small hole in the lid.

4. Place in refrigerator

My question is about step #3. Why is it necessary to punch the hole in the lid?

Thanks!