Why you need a Microsoft Account with an Office for home product
Using a Microsoft account and linking it with your Office for home product connects you to various Microsoft products and services, and makes it simple to reinstall Office without a product key.
Note: Linking a Microsoft account with Office only applies to Office 365 Home, Personal, or University, or Office 2013 or newer versions such as Office Home and Student, Office Home and Business, Office Professional and some versions of Office Professional Plus.
A few things to keep in mind:
A Microsoft account is required to install and activate Office versions 2013 or later, and Office 365 for home products.
You might already have a Microsoft account if you use a service like Outlook.com, OneDrive, Xbox Live, or Skype; or if you purchased Office from the online Microsoft Store.
If you don't have a Microsoft account yet, you’ll be asked to create one the first time you install or activate your copy of Office.
When you link your Microsoft account to Office, it means anytime you want to install or reinstall Office you won't need to re-enter your product key, or hang on to an installation disc or thumb drive.
If you use Office 365, you can add your credit card information to your Microsoft account and turn on recurring billing so Office is always available.