Besides the (correct!) sibling comment: the idea here is to provide underlying primitives which can then be used to express UI in a variety of contexts, first of all native UIs but (as the post notes at the end) also including the DOM. For example, people have implemented experiments which use SwiftUI for authoring HTML, and you can use React to author native UI (React Native, but also e.g. the Raycast widget system), text UIs, etc.

So that you don't have to ship an entire web browser for your application.