why it is used
3d modelling is used to either to see if something will work in real life so they don't waste time and can change the design quickly at any time during testing.If it was done without 3d modelling it would be very expensive or dangerous if it didn't work.
what its used for
3D modelling is used to make cars,rollar coasters,kitchens and much more they use it with cars to test iff it will work because it will be expensive to make and if it doesnt work it will be a waste of money.They use it with rollar coasters to make sure it will stay together because if it falls appart it could be dangerous.They also use it to make bridges to see if it can hold the weight of cars and trucks and make sure its safe. Finaly they use it for kitchens to make sure everything will fit in and have enough power.