A P4Web Viewer is used strictly as a depot browser and document server. No user authorization or password is required to connect to a P4Web Viewer; it can be accessed by anyone. However, you can't sync or edit files using a P4Web Viewer, nor can you edit client specs or user specs. In fact, you can't do anything that changes the state of files or metadata in the Perforce depot. Therefore, many of the shortcut buttons and menu choices you'd find in the control panel of a P4Web Helper won't be available on a P4Web Viewer page.
For information on starting up a P4Web Viewer, see How to Start P4Web.