Why Does Discoveries Give the Clothing Measurements?
Clothing sizes are no longer standardized in North America. You might wear four or five different sizes, depending on the brand. So if you know your own body measurements, you can compare to the garment’s measurements and judge more accurately how something will fit you.
Non-stretchy items generally require 3 to 4 inches (7 to 10 cm) extra fabric for ease of movement. So you should measure that much smaller than the garment. For example, if your chest measures 36”, then the garment chest measurement should be 39 to 40”, unless you want a very snug, constraining fit (often very nice in close-fitting jackets, or non-stretch jeans).
Stretchy items are measured before stretching (unless indicated otherwise) and can generally be worn when your body measures up to 3 or 4 inches larger than the garment. (But if you want a looser fit, then you would want the garment to be bigger than you are.)
You might be interested in reading How To Measure Yourself.