The easiest browser tests live entirely inside one tab.
Open a page. Fill in a form. Click a button. Check the result.
Unfortunately, many valuable user journeys do not stay inside that neat boundary.
The browser itself participates in the workflow. It stores sessions, asks for permissions, handles downloads, opens deep links, restores tabs, applies location settings, and moves keyboard focus. Email may become part of authentication. A push notification may become the entry point to the application.
At that point, you are not merely testing a web page. You are testing an agreement between the application, the browser, the operating environment, and several external systems.
