Whats is the function of the metal poles-tubes-bars on the sides of early 20th century battleships?

Answer 1

If you mean the poles fitted at an angle along the sides of the hulls of battleships, these are supports for anti-torpedo nets.

When a ship was at anchor outside of a harbour it was liable to be attacked by submarines or destroyers, so the crew would swing out the poles and unfurl the heavy nets attached. These would suspend below the waterline, hopefully to stop a torpedo.

They were gradually removed when it was found torpedoes could be fitted with net cutters and also the effort to swing them out and back in again when wishing to move the ship was very time consuming.

In WW2 some merchant ships were successfully fitted with nets for use while sailing but they tended to slow the ships too much.