Why do I have to pay for all apps including this

Apple has it set up so that you must have some payment method on file to download apps – even free ones. If you want to know "why", you'll have to ask them.

I believe that in some parts of the world, the payment method can be an Apple Balance, loaded from an Apple Gift Card that you purchased at a retail store.

Note that you will not be charged for downloading a free app.

However, many "free" apps these days have artificial consumables that cost real money in the form of in-app purchases. I prefer the old arrangement when third-party developers charged one-time fees for apps, up-front, rather than trying to get you to install vending machines on your device.