Why were the rowers chained to the benches?

I can’t believe that the rowers in the galleys, at sea, were tied with chains. They could not escape because of the danger of drowning, there was simply no point. They, often, were even fed no worse than the rest of the crew of the galley – after all, such hard physical work required strength.
Some historians believe that galley rowers were a kind of antique naval special forces.
If they were chained, in some cases, then, I think, it was only in the case when the rowers were slaves and the owners were afraid that the rowers were organizing a conspiracy and a rebellion would begin.