It's useful in following conditions:
(i) when you want to know the valency to male compounds
for example :
when H2 and O2 react in normal conditions they form H2O, amd here we needed crossmultiplication of their valencies.
(ii) To know masses while calculating mass number.
For example In case you wanna find mass of 100 molecules of H2O.
(iii) To find electronegativity (to determine type of bond formed).
For example: which kind of bond Hydrogen and Clorine will form.