This is part of the overall idea of checks and balances that is one of the major bases of our political system. The system was set up like this so that no part of the government would be completely independent of the other parts. That way, the parts of the government can stop one another from doing things that tend towards tyranny.
By making it so that the Senate and the President both have to concur, the framers were trying to make it so that neither the President nor the Senate could have too much influence over the judges. They were trying to make sure that the President could not get into office and start appointing judges who were for him but who were likely to do really bad things once in office.
If it were not for this, the President could come in and appoint people who were incompetent but were his friends. Or he could appoint people who were really radical in terms of their beliefs. By making the Senate approve the appointments, the Framers prevented this from happening.