Why did Bill Nye pressurize the giant "cloud bottle"? What purpose did this serve?

Water molecule is present in air these airborne water molecule are called water vapor, when the molecules are present in air they don't normally stick together as the space is too much.

Bill Nye took a giant bottle and covered it with a stopper and started pressurizing it with a pump to make the molecules come closer and become compressed also it increase the temperature even slightly and that warmth will cause the liquid stick with the smoke particles to go back to forming as vapor.

After that he removed the lid then the air inside the bottle is spread out and it will eventually cool also the kinetically energy is also reduced and that cooled air sticks with dust particle which now forms cloud.

This experiment shows how the clouds are formed and the science associated with it.