Why negotiate a peace treaty when nobody is at war?

Why did these countries need a peace treaty when none of them was at war?

The aim was to prevent a future war and reduce naval competition, leaving governments more money for other priorities.

The treaty's purpose was preventive. Its supporters expected equal British and American naval forces to remove a reason for conflict. Matching fleets would also reduce pressure to keep building against one another. Both governments wanted to spend more money on other priorities.