The idea of global navigation is to link to resources that are common to everyone in your organization, such as human resources or employee policies. Global navigation, as a native feature, is not yet available in the SharePoint modern experience, but can be created using SharePoint Framework Extensions (SPFx), creating an intranet home or navigation site as a launchpad and including a link to it on local or hub site navigation bars, or using a single hub for all of an organization. 

The size of your organization will drive some of these decisions. As the modern SharePoint experience is quite flexible, and as new features such as global navigation become available, updating hubs or redeploying SharePoint Framework Extensions requires much less administrative effort.