useDocument() returns the current document. In the browser useDocument() is identical to document (as browser only has one document). On the server, there is no global document; therefore, referring to document is undefined. Additionally, the server may be processing many different requests and therefore may have more than one document at a time. useDocument() on the server returns the document associated with the current application.
IMPORTANT: Qwik currently uses domino on the server and useDocument() returns the domino instance. This is a temporary solution until Qwik switches to string concatenation on the server, in which case the useDocument() may be deprecated.