Among other things, metrics can help managers track design progress and support decisions about when to release a product. Usability can be measured, but it rarely is.
Metrics are expensive and are a poor use of typically scarce usability resources.
Most companies still under-invest in usability. With a small budget, you're far better off passing on quantitative measures and reaching for the low-hanging fruit of qualitative methods, which provide a much better return on investment. Generally, to improve a design, insight is better than numbers. However, the tide might be turning on usability funding. I've recently worked on several projects to establish formal usability metrics in different companies.