Well, let me give it to you square and simple.There isn't that much to say about it, but to elaborate, metal plating can strenghthen, flex, or even beutify any other metal. It also conducts electricity.
To give an example, there's a metal structure. made out of really sturdy material, built to last, but the only problem is, it's ugly. You can't just use a different material, it wont be as strong. So what you do is you plate the stronger aterial with a nicer looking material. Then you've got a nice sturdy structure, and a nice, pretty appearance. This is one of the most common uses for metal plating. To beutify a substance, and also, to strengthen a substance.
Also, as I said above, certain metals conduct electricity. For instances where a link in electricity is needed, plating can make the required link. The metals used in metal plating are also usually really resiliant to wear and tear. So after years of doing the required job, the object will still be usable.
If you look at a quarter, just an adverage US quarter, you will notice that on the side, there seems to be a brownish line inbetween the sliver. This brown line is a plate of copper. It's there to stop corrosion, or to stop the two silver plates from mixing and deforming. That's pretty much all there is to say about why metal plating is used.