It's definitely not necessary, I've done dozens of cars without doing this. But I will say this, it does eliminate the little pools of candy that can give you tiny darker spots in the paint, if you do seal the metallic base first. I know this because when I do graphics over candy, I DO seal it first with clear, then wait a day or 2 and sand, then apply graphics and clear topcoat. I don't think it looks deeper, but it does get rid of the small dark spots.
If you do have too rough a finish on your basecoat, it would be a good idea to do this. Or if you use a large metalflake for a base.