Fuel Injection - explain this to me like I am 6 years old

A lot of the time its cheaper just buy the extra gas.
INDEED! You can buy a LOT of gas for $2000. Will fuel injection make your car start better, improve throttle response, and most likely improve gas mileage? Yes. But ultimately you have to ask yourself if it is worth the cost to you. Plus, do you only drive it occasionally, or is it a daily driver?