In Article 1, Section 8, the Constitution lists many powers of Congress. Are all these powers necessary?
Congress is supposed to declare war, not the President. The fact that Congress has ceded its power in this regard to the Exceutive Branch constitutes one of the more serious breaches of the Separation of Powers. It is not beneficial to the US or the rest of the world to have the Executive Office as the declarer of wars and as the Commander in Chief. This represents a regression towards Absolute Monarchy, where the king was judge, jury, and executioner. Moreover, if Congress would commit to a conflict, they would work to finish the job if war is declared, or hesitate to declare war in the first place. For fear of losing re-election, no one wants to back any war, should it go badly. Congress must begin again to live up to its responsibilities as outlined in the Constitution.
That depends a great deal on what you, personally, think. However, the Framers of the Constitution were concerned because the Articles of Confederation (the first constitution of the US) had not given the national government enough power. Therefore, they wanted to fix that by making it quite clear that Congress had a great deal of power. They also thought it was necessary to list powers that Congress had just to be sure that no one would misunderstand what they were trying to do.
As I look through the clauses of that section, I personally do not see any powers that seem unnecessary (although Congress is not likely to be granting any "letters of marque" to authorize American privateers anytime soon...).