The reason why we need your consent is new and very strict legislation called General Data Protection Regulation (GDPR). Since the 25th of May 2018, all European Union based apps have to ask you for consent to process your data before you can start using them or they can face legal charges.
First of all, when you are using the app without buying anything, you are an anonymous user and we don't collect personal data about you. The only data we gather is strictly technical. f.e.: Device ID is an anonymous technical data that every app collects - it is the very basic identifier to track bugs, crashes, optimize the app's behavior for different iOS devices, and to resolve potential technical problems you may encounter as a user.
Once you purchase some services it is essential that we identify your Apple ID so that we can deliver the purchase or service. The new legislation mentioned above considers this info as sufficient for the identification of a natural person and therefore we need your permission. The fact that many apps ignore the law doesn't mean they don't collect data about you, they just don't let you know.