Q: why do I need a credit card number

You are not required to.  You can select "none":

Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

You don't...  This document describes how to use Apple's digital media services without one.

Create an iTunes Store, App Store, or iBooks Store account without a credit card or other payment method - Apple Suppor…

As an additional tip, when you make occasional purchases, a good way to do it is through an iTunes gift card.  You can buy one for yourself at a desired value, and enter it as "store credit."  The amount appears at the upper right corner of iTunes window when you are in the iTunes Store screen.  You can use this credit for Apple's digital media and services.  For example, songs, audiobooks, ebooks, apps, movies (including rentals), TV shows, and subscriptions (such as iTunes Match, Apple Music, and Netflix or Hulu on Apple TV).  The best part - you can often buy them at a discounted price, such as $40 for a $50 gift card.  Store credit is used first, before other payment methods.