Apple created the Apple ID system originally in order to improve customer experiences on their online store. You could get an email copy of your receipt, track your order, file a rebate, etc. Then, Apple launched the iTunes store in 2003 which leveraged your Apple ID to purchase music.
By 2008, we were using Apple IDs to purchase movies, television shows, music videos, audiobooks, and of course, apps! In 2010, Apple added ebooks to the mix. By late 2011, Apple was using Apple IDs as the user login credential for freely available iCloud accounts. Apple IDs have come a long way (at inception, they did not require an email address, unlike today).
As much as Apple’s Apple ID system has its faults, it is still an effective way of tying your data to you! And in an ideal world, every individual has their own Apple ID. In the real world, customers often have more than one Apple ID or some customers might even share an Apple ID. It is important to remember that due to how Apple IDs are used, sensitive and important data is associated with them.
An effective way to understand the way Apple IDs work is to understand the two primary ways they are used: for personal data and purchased data. Today, a family can share an Apple ID and use it on Apple’s iTunes, App, and iBooks Stores to purchase content once and access it on anyone’s device.
However, it would be ill advised to use that same Apple ID for any one member of that family for their personal iCloud account. That’s because every other member of that family is only a few taps away from syncing their personal data with yours. Therefore, it’s a common practice to share a single Apple ID with members of a family for purchased data while every individual has their own personal Apple ID for iCloud synchronization of personal data.