Why is motor oil sold in different viscosities?

I looked it up, and I don't quite understand. If you could break it down, that would be great. I need it for homework.Why is motor oil sold in different viscosities?
As oil cools down it gets thicker and when it heats up it gets thinner. When oil gets too thick to go between parts it should be lubricating there is no lubrication. This is why in the old days you put 10 or 20 weight oil in a car up north during the winter and 30 or 40 weight oil in the car down south in the summer. In modern times they have multi-viscosity oil. 5 w-30 means it will flow like 5 weight oil and protect like 30 weight oil. That means it will get into all the places it needs to in cold weather and still protect in the warm weather too. An added benefit is it reduces waisted energy needed to move heavier oil when it isn't needed.Why is motor oil sold in different viscosities?
because it can be used for different cars