Why experimental Turtle submarines had to be built

Why were American Turtle submarines built if they were never going to be useful?

They seemed plausible, and only a real test could reveal what worked in an era without modern simulation.

Turtle seemed plausible within the knowledge available during the American Revolutionary War. Its inventors could not use computer simulation to settle the design's performance in advance. Building and testing the submarine was therefore the available way to learn whether it worked. A failed trial could reveal a problem that an apparent first success concealed until the design faced different conditions.