We need the navy to patrol the seas in the world to keep it safe for commerce. Otherwise, you’re right.
Actually, we don't.
The U.S. is theoretically more autonomous, more self-sustaining, than any other nation on Earth.
We choose to trade, because it boosts profitability.
But more than any other nation on Earth, the U.S. could completely sever ties, cease trading with all other countries (save Canada and Mexico - for which we don't need a Navy), and survive just fine!
We patrol the seas for the benefit of other countries (which indirectly benefits us).
Regards,