Someone enlighten me!

I'm not an engineer, but since I've been driving i've noticed a few things.

If I put my car in 3 and punch the gas, the car seems to accelerate better w/o spinning the wheels (losing traction). Why is this?

What's the fastest way to get off a stop, start with the shifter in 3 or lower? if so, why is this?

I'm not interested in doing any racing, I just want to know why it works or if i'm imagining it.

Thanks you guys.