I think that the phrase necessity is the mother of invention means that without the need for things people wouldn't invent anything. That's why when you need something, you use it, (or invent it) and that's why necessity is the 'mother' of invention.

For example, people needed to move from one place to another using something that's faster than walking, so they invented the wheel, and later on cars.

People wanted to be warm when it was cold out, so they invented a way to get fire. In this case, it also helped them cook food.

Humans felt that they needed a way to communicate with people far away without sending fire messages, so they invented the telephone.

When we feel that we have a need for something, someone comes up and invents it. And then we use it.

http://www.chrismadden.co.uk/yah/invention-of-wheel.gif