What is 3D modelling?

and why is it used?

Why?

3D modelling is used because it's a good way to plan creations. So say you're building a rollercoaster, you can easily make changes or improvements and correct mistakes. Another reason that's fairly exclusive to the scenario of rollercoaster making, you can simulate how it'll feel for a human. If you use human subjects to test a rollercoaster, something could go wrong. Also, once you've made the rollercoaster, if you've made a mistake you have to completely start again. In turn this saves money.