OK Karsten, I will answer your (fairly dumb) question. There are many reasons why gear inches is a useful measurement.

1. If you want to gear up or down a certain amount, often adding or subtracting a teeth from your existing setup (the bonehead method) can have undesirable consequences. It may throw off skid patches, etc.

2. What if you want bigger rings for better wear, but about the same development as your current gear?

3. For different people to be able to meaningfully discuss gearing, a universalizable standard is needed. Gain ratios are a tiny bit better, but crank length makes so little difference that you can get along without it.

4. Say you need a magic gear, and you want it to be well suited to your commute.

5. Maybe you want a setup with all prime numbers, and you want it to be suited to your commute.

6. Maybe you want a setup with all odd numbers, and you want it to be suited to your commute.

7. Maybe you want a setup with all even numbers, and you want it to be suited to your commute.

I could go on; there are all kinds of conceivable situations where calculating gear inches is useful. Knowledge is power. I am bewildered that you do not seem to understand this.