Because some apps are free but have provision to make in-app purchases.
To verify that you are who you say you are in the country you claim to be in
so that you cannot get apps that are not available in your country.
Purchase an iTunes card and redeem it in your account to give yourself a positive
balance; then you will be able to download free and paid apps.
From post by TexasMacMan:
How to Get Apps From the App Store Without a Credit Card
Creating an iTunes Store, App Store, iBookstore, and Mac App Store account without a credit card
If None is not available - On your computer launch iTunes and click "iTunes Store" in the left navigation pane. Click the "down arrow" next to your name at the top right side of the page and click "Account." Enter your username and password and click "View Account" to log into your account information. Next to your Payment Type, click "Edit." Select the "None" button and click "Done." Confirm that your card has been removed by returning to the Apple account information screen. Under Payment Type, it should say that there is no credit card on file.
Here's the deal: There is only one way to create an Apple ID & not have either a credit card or iTunes gift card associated with the ID. You follow the directions here:
This can only be done when first creating the ID. If you have an existing ID, you MUST have either a credit card or an iTunes gift card associated with your account. You will not be able to select "none", unless you redeem an iTunes gift card.