There are two aspects to this question.
First, we must look at why anyone is given the right to confirm or reject the people that the president appoints. Why aren’t presidents allowed to appoint whoever they like? The reason for this is that the Framers of the Constitution did not want any part of the government to have too much power. They wanted to have checks and balances so that no part would be too strong and, therefore, be able to tyrannize the people. For this reason, they did not want presidents to simply appoint whoever they wanted with no curbs on that power.
Second, we must ask why the Senate, and not the House of Representatives, gets to do this. This is because the Framers intended the Senate to be the more mature and more important of the two houses of Congress. Senators were originally not directly elected. They had to be older. They got to serve terms three times as long as those of representatives. In short, they were supposed to be the more important house. For this reason, the Senate was given the responsibility of checking the president’s appointment powers.