Such 'back' links are not useful.
As stated in the question, they merely repeat a function already available in the browser. More importantly, however, unlike the browser back button, users may be surprised by its behavior. For example, they might not expect to be sent back to Google when clicking a link on your site if they arrived there via a search. Such links do not add anything useful and can be confusing,
There is also considerable amount of UX research that indicates that most users are comfortable with the browser back button. So it is also redundant. But that is a lesser sin than being potentially confusing.
However, if the concept of going back is important to your navigation, there are numerous other approaches available that will not risk confusion, and do not depend on your user entering via the "front door" (so to speak). 'Breadcrumbs' being the most commonly used tool.
Breadcrumbs enable users to go "back" in terms of the sites hierarchy even if they enter via a deep link. They also provide immediate access to different levels of the site hierarchy without having to step through each layer. Finally, they also inform users of the site structure and thus facilitate discovery.