A smaller size battery can be converted/ upsized and used as a larger battery in case of an emergency situation.

Ex. Clock ran out of battery and required a size D battery; however, you've got only an AAA at hand. Simply slide an AAA into the shell and it can now work as a size D.

For household use, only a smallest AAA-size battery will be needed, it won't be necessary to buy batteries in variouse sizes: thus saving money and helping to protect for a better environemnt.