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 and the rattlesnake being eaten.
Various rules have evolved to prevent such misreadings and to speed readers along through complex grammatical structures. Those rules are detailed in this section. (Section P2 explains when not to use commas.)