What is the application value of reciprocity rate?

Reciprocity rate is a "means" or "method" to calculate the amount of light entering. How to measure by value?

Just like 2+2=4. Just some things here. Use it when you need to calculate how much two apples and two apples are. You can't say that 2+2=4 has any value. Similarly, you can't say that pi has any value.

It's just an existence. Use it when you need it. You don't need to remember when you don't use it.