Here's what happens at most startups: someone sets up analytics, everyone tracks everything, and now you have 47 charts that nobody looks at. Page views. Button clicks. Scroll depth. Average session duration. Bounce rate.
None of that tells you if your product is working.
Good metrics are the ones that change your behavior. If a number goes up and you do nothing different, or a number goes down and you do nothing different — it's not a metric. It's decoration.