Write a paragraph in which you name one power that the Constitution denies to states, & explain how the nation benefits from that restriction.

To me, there is no question as to which power denied to the states is most important (assuming you are talking about the United States).  The Constitution says that the states may not regulate interstate commerce.  This is hugely important because it helps (and has helped) the US economy.

Before the Constitution, states could and did erect tariffs and other trade barriers against each other.  This made it so that various states became like separate countries in terms of trade.  It meant that people from one state could not trade easily with one another.

Today, economists believe that free trade is good.  We have things like NAFTA to give us access to larger markets.  If the states were allowed to have trade barriers, we would not even have free trade within the US.  That would weaken our economy because our companies would not be able to have such big markets -- they would only be able to sell easily within their own state.