This will help you to save money if you have some batteries of a different size that you bought for some other product that you no longer own. No one wants to waste them.
省钱了：如何不守规矩替换不一样大小的纽扣电池？

Another benefit is saving time: learning this probably takes about as much time as going to the store to get a new battery, and is a skill that can be applied more than once, and may be useful for items other than key fobs as well. If you already have these other (wrong-sized) batteries on hand, you can also avoid adding to e-wast (it takes more time and care to deal with battery waste).