Other than to land the last hit and capture the city, or for the melee seige units obviously. A 20 pop city with a castle has about 45 strength. That will eat riflemen alive and even units stacking bonuses to get 45 strength will kill themselves trying to melee it (and get one shotted by the city and its defending units next turn). Attempting to use sea beggars with logistics + coastal raider 3 resulted in the above happening. I would attack twice, get brought down to less than 50% hp by my own attack, deal neglible damage to the city, then the city would one shot it next turn. I gave up and went back to frigates. Naval wise, theres also no reason to build more than one or two melee units as ranged naval units massively outdamage melee ones. A frigate has more ranged strength than a privateer, and the same combat strength. The frigate wont take damage when it attacks, so the privateer is at a disadvantage from the beginning. It gets worse when destroyers have no upgrades but battleships upgrade to the vastly superior missle cruiser. Ground ranged units have less combat strength than melee ones, but this doesnt apply to naval units for some reason.