3D modelling is for planning out what will happen on computers. It's there so you don't have to waste time building a car that will go wrong. Computer generating is the next step forwards as it helps you to develope and test your product easily, without the hassle of making and creating a prototype. Ofcourse you will still need to make a prototype, but it will already be what you want it to be at no expense.  ( sorry about the ' , it won't let me put in apostraphes :/ )

How is 3D modelling useful?

3D modelling is used for many things such as designing cars and rollercoasters so you can test them in complete safety without having to spend loads of money on building it. It is also used to create movie characters in animations such as wall-e, or sully from monsters inc. and wall-e.