"The comma was invented to help readers. Without it, sentence parts can collide into one another unexpectedly, causing misreadings.
confusing If you cook Elmer will do the dishes.
confusing While we were eating a rattlesnake approached our campsite.
Add commas in the logical places (after cook and eating), and suddenly all is clear. No longer is Elmer being cooked, the rattlesnake being eaten..."(145)
I will share some of the rules Diana Hacker points out later.