This feature allows for multiple custom element definitions for a single tag name to exist within a page to prevent custom element name conflicts when a web app uses libraries from multiple sources. This is achieved by allowing user code to create multiple custom element registries and associate them with shadow roots that function as scopes for element creation and custom element definitions.
It's quite common for web applications to contain libraries from multiple sources, whether from different teams, vendors, package managers, etc. These libraries must currently contend for the global shared resource that is the CustomElementRegistry. If more than one library (or more than one instance of a library) tries to define the same tag name, the application will fail. This feature solves the problem by using ShadowRoots, which already function as encapsulation boundaries, to also function as scopes for custom element definitions.