You shouldn't need one. But if you buy this software, you won't be able to install it until you create a Microsoft account (or sign into an existing one). You are being forced to give up some privacy in order to be able to use the software you just bought (which looks to me to be a violation of the CCPA).
A Microsoft account is an email address and password that you use with Outlook.com, Hotmail, Office, OneDrive, Skype, Xbox, and Windows. As part of purchasing Office, you’ll be prompted to sign in with your Microsoft account. After redeeming your purchase at setup.office.com, your Office product is added to your Microsoft account and you’ll use this account to install or reinstall Office from account.microsoft.com/services. To learn more about the benefits of having a Microsoft account, visit https://go.microsoft.com/fwlink/p/?LinkID=403717
CCPA begins at the start of 2020 as I understand it. Also Microsoft is making it very difficult to create a standard user account when you set up Windows. They are doing everything they can to get you to sign up with an account. IMHO this is very predatory and should be illegal. I would go with open source if you can.
A Microsoft account has the benefit of accessing Outlook email and docs saved to the cloud accessible to all devices using that login. Any purchased software can also be managed by the account. For example, of your hard drive crashes and you need to reinstall a Microsoft product, you can go to your account to reinstall and also retrieve your license keys.