ELI5: Why do I need to pump the gas on my old 80's car before I start it?

I want to know what I am doing mechanically and why, please. I don't want to just go outside and pump the gas every morning because someone told me to do it. I'm 5 and I'm already showing signs of independence (It's a provisional driver's license.)

FYI: I tried a few youtube videos but they all assume working knowledge of the rest of the car/motorcycle/lawnmower. I have none of that.