What is the practical significance of the law of constancy of the composition of matter?

The law of constancy of matter: any definite chemically pure compound, regardless of the method of its preparation, consists of the same chemical elements, and the ratios of their masses are constant, and the relative numbers of their atoms are expressed in whole numbers.

In practice, this is used to solve problems. What chemical elements enter into the reaction, such will be in the reaction products as part of other compounds. The mass of substances that enter into a chemical reaction will be equal to the mass of the reaction products in closed conditions. Thus, the practical yield of any chemical process is calculated.