Users, who sincerely use Apple products and its software, keep only one ID to access multiple channels like iTunes, iCloud, iBooks and others. This gives them ease of access as they don't need to remember IDs and passwords of more than one email address. This is a good practice if you are the only user of Apple Music and other sharable content.
There are other users who share content on iBooks, Apple Music, and App Store; and for them, sometimes more than one Apple ID is necessary. They use different emails to access iTunes and iCloud. At this point, they need to change their iTunes account at regular intervals on their iPhone and iPad.
Here is a simple method you can follow to change your iTunes account on your iDevice.
How to Change iTunes Account on iPhone or iPad
Step #1. Open Settings app on iPhone/iPad from home screen.
Step #2. Tap on iTunes & App Store.
Step #3. Tap on your Apple ID.
A menu will appear on your screen with three options: View Apple ID, Sign Out, and iForgot.
Step #4. Tap on Sign Out.
Now you can sign in with a different Apple ID.
Please note that you can use not more than 5 computers and 10 iOS devices you have authorized on the same iTunes account at the same time. Moreover, you cannot change iTunes account only after 90 days; if you change the account in between, your iTunes Match and iTunes in the iCloud will not work until the next 90 days.