Suggest why platinum is used as a catalyst even though it is a very expensive metal.?

This is because the catalyst is chemically unchanged at the end of the reaction. Therefore, the platinum catalyst is not consumed and can be quantitatively re-used.

As with any engineering design problem you have to balance cost versus effectiveness. In this case platinum was the best choice when you consider these considerations. 

Because it works!

Because molecular bonds are easily formed and broken allowing platinum to return to its original state with a minimum of chemical actions.