Why do people yell at dogs?

I always wonder why people yell commands at dogs, all the time. As if shouting will make a dog obey when even adult humans who know the language sometimes don't. A few months ago, I visited a friend in the US who was keeping his neighbour's dog while they were on a holiday. He kept yelling and the big male labrador toyed with him by pulling at his leash while they were out for a walk. The louder my friend shouted the harder the dog tugged. My friend almost tumbled. He asked me to watch the dog for a few minutes while he entered the shop. I wondered if the dog would tug at the leash. He looked at me. I whispered, "Sit." The dog sat. Ten minutes later my friend emerged. I handed him the leash. And the dog took my friend for a run.