Why does Duracell company sell batteries in a package that includes a tester?

Explanation

A tester in a duracell batteries package is a liquid crystal display the colour of which changes with temperature. It is used to test the strength of the battery. If the battery is fresh, the current supplied by it is strong, which will produce a large heating effect on the liquid crystal, resulting in a larger change in colour of the crystal. If the battery is weak, the change in colour of crystal in the tester is small.