“It has been found experimentally...”
The significance of this discovery remained unclear until February 1953. Thats when Watson figured out how DNAs four bases paired with one another. By fiddling with cardboard cutout versions of the base molecules, he discovered that adenine always pairs with thymine, and guanine always pairs with cytosine. Now Chargaffs finding made perfect sense to Watson and Crick: If every adenine and thymine are paired in DNA, there must be an equal number of these two molecules. The same goes for guanine and cytosine.