Metrics are useful little tools.
Metrics might not seem useful when you are working alone. When you're working alone, you know how you're doing. You have your hands on the work, all the time. Looking back at the day, or the week, you can feel whether you made progress. Looking ahead at your goal, you can feel whether that progress was enough, or if something needs to change.
Now throw a team into the mix. Your teammates are working and you are not touching it. Even if you are the lead, you don't touch all of it. Not if you're doing it right, anyway. And if you're the project manager, well, you might not even be touching all of your own team every day. Your work also involves touching other teams, whose people and work you aren't touching either.
So you start measuring. Story points, features, bugs opened, bugs closed, test count, lines covered, velocity, backlog size, ideal days, person hours, burndown. The numbers start rolling in. You touch them, and you get a feel for things again. Numbers are moving up and down and you can feel whether they're moving in the right direction and fast enough.
But you're still not touching the work. You're touching the metrics: a set of narrow, fuzzy, possibly staged views of certain aspects of the work. You're touching a tool. Tools wear. Tools break. Tools become obsolete. Sometimes, tools just get in the way.
If the metrics are helping you ship, then the tool is doing its job. But the metrics can't tell you how good the metrics are. Never forget, your job is not to deliver the metrics. Your job is to deliver the deliverable. Your job is to ship the product.