What does it mean to change gears in a car?

I'm taking a Summer class at my university called Behavioral Analysis.

We have this term called a "behavioral antecedent" which basically is anything that signals you to perform a certain behavior such as a green light telling you to go or your mom's voice calling your name, signalling you to come to her.

This guy said that a behavioral antecedent would be when your "engine MPRs start to rise around 2500 to 3000, that's an antecedent for you to upshift."

Most of us didn't understand what that meant. This one girl said, "What are you talking about, the 2 and 1 on a gear shift is just there for towing, you're supposed to keep it in drive, numb nuts. This was never a thing."

That was quite rude, but I personally don't know why you would switch gears in a car. Why do you do this? I personally just keep my car in drive. I currently drive a 2007 Honda Civic.

Read more: What does it mean to change gears in a car?