It establishes the expected behavior. You're absolutely right that it doesn't teach sincerity, but that's not the point. It builds the habit, and it forces acknowledgement that another party was harmed (even if they lack the empathy to care about that yet, empathy starts with awareness).

Sincerity is largely learned by example. It's important for parents to model sincere apologies when they screw up. To each other, to their kids, etc.