Multiple Apple ID

The email addresses @me.com, @mac.com, and @icloud.com are all from one iCloud email account. If you have the first two you had these emails as part of Apple's precursors to iCloud and you carried these addresses into iCloud.

Apple explains this in more detail in the document iCloud: About your @icloud.com, @me.com, and @mac.com email addresses - Apple Support

Is this what you mean by using several different IDs? These old addresses have become alias addresses for the current iCloud email address.

Why can't Apple allow you to merge two Ids into one account? I have me.com, mac.com, and icloud.com. The first was Mac. Then they came out with the Mobile and followed that with icloud.com. But if are using just one and delete the others, you lose App purchases made on them.

Apple IDs cannot be merged or combined.

You might be able to share bought content across your multiple Apple IDs with Family Sharing. See What is Family Sharing? - Apple Support

Thank you , I never thought of trying that.