Answer:To break a tie vote on a bill
The president of the Senate is allowed to vote on legislative matters in order to break a tie vote on a bill
Explanation; Under Article I, section 3 of the constitution, The vice president of the United states shall be the president of the state, but shall have no vote; unless they be equally divided.
The president of the senate presides over the senate only on ceremonial occasions or when a tie-breaking vote may be needed.