Q&A: What did more oarsmen give a warship?

What did galley types larger than quinqueremes gain from additional oarsmen?

They gained power and acceleration, especially for pulling out and striking, while enabling a larger and heavier ship.

Rowing power affected more than top speed. Extra oarsmen helped a galley pull away, accelerate into an attack, and move a larger structure. The heavier hull consumed some of that added power, so numerical class cannot be read as a simple speed ranking.