what do i need an apple id for

For everything you do with Apple (including logging into this forum). See the following from How to create a new Apple ID - Apple Support

An Apple ID is the account you use to access Apple services like iCloud, the App Store, the iTunes Store, Apple Music, and more. It includes the email address and password that you use to sign in, and all the contact, payment, and security details that you'll use across Apple services.

No matter where you create your Apple ID, just remember to use the same one to sign in to every Apple service. That way, you can access and manage everything with a single account and password.

Basically an AppleID is Apple’s universal login account for all their services.

See this document for the list of services and features that require your AppleID to login and use -> Where can I use my Apple ID? - Apple Support