Congress has "the power of the purse," which is the power to tax, borrow, and spend. Why is this an important power?

This power is important for congress to have because money keeps the government going. Letting only Congress use this power ensures that the country's money Is used correctly, being that it is decided by a vote.

No government is run without finance and this falls in the hand of the congress. The treasury of the nation is handled by them.