You should also connect to the Internet regularly to keep your version of Office up to date and benefit from automatic upgrades. Internet access is also required to access documents stored on OneDrive, unless you install the OneDrive desktop app.
For Microsoft 365 plans, Internet access is also needed to manage your subscription account, for example to install Office on other PCs or to change billing options. However, Internet access is required to install and activate all the latest releases of Office suites and all Microsoft 365 subscription plans.
You don’t need to be connected to the Internet to use Office applications, such as Word, Excel, and PowerPoint, because the applications are fully installed on your computer.