“In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meager and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be.”
- Lord Kelvin
Kelvin's quote is equally applicable to project management as it was to the physical sciences. When we hear of a suggestion for improvement and ask about the evidence of success, the response was usually just try it. This was common in the early days of agile software development. Just try it, works fine if it's you money and your own customer. But what if it's not your money? What if the customer is expecting that you have the methods in place to produce what was promised in the contract.
Then you probably need a measurement system - of some sort - to confirm that your promises are being fulfilled.