I’d like to start with a brief test. All those who have more than one Apple ID, please raise your hands. Now look around you; if you’re in a group of people who use Apple products, you’ll see a lot of hands in the air. And if you’re not, well, you can lower your hand now…
If you’ve been using Apple products for a while, you may have multiple Apple IDs. One might be a user name, and another an email address. Or you may have set up one Apple ID for the iTunes and App Stores and another for your personal data, such as your email and other iCloud services. (And this is fine; Apple even explains how to do it.)
Some people may have set up a second Apple ID because, for some reason, they couldn’t access the account with the first one, and simply gave up. Or they used an email address they no longer use, and created a new Apple ID with a more current address. In either of these cases, they cannot download apps or media purchased with the older Apple ID.
Read the rest of the article on Macworld.