Sharks eat sick, injured or weak fish because they're easier to catch. This way, other fish don't get sick and weak fish don't reproduce, making more weak fish. It's an important job, but it doesn't make the shark too popular. A dog barks or growls to let you know that it's upset. It's giving you a chance to go away.
Some sharks do the same thing.
They can't bark but they put on a swimming show. The shark probably prefers not to have to go to the trouble to do anything like chase or attack. The liver stores fatty acids that provide energy when the shark cannot find food. A shark's liver also provides buoyancy, especially important because the shark does not have a swim bladder, which keeps most fish afloat. Sharks are long-lived.