The presidential veto a powerful tool because if the president refuses to approve the bill, then that act mentioned in the bill is not passed even after all the approvals.
Explanation:
In US Senate, when a bill or any sort of joint resolution is passed by the house, it goes to president for approval. The act, in order to go in effect, president must agree to the same and have 10 days to sign.
If the president declines, and prevent the law to be enacted, it is known as a veto. If the president vetoes a bill, it won't be approved, even after each and every approval. Hence presidential veto is a very powerful tool.