Toby Perhaps it was intended just as a UI cue for the slide-out discussion list?
I think this is really what it does, isn't it?
Issues with the Back Button (from UI Consistency thread)
So the thing is... the 'Back Button' isn't actually a back button...
Its literal meaning is: 'from a Discussion, take me back to the Discussion List which I was previously viewing'
(and forums do usually carry this particular function; it's usually labelled "back to xyz forum").
Explanation
If you were viewing say, the Dev Tag, and then open a Discussion in it, then hit the 'Flarum Back button', you get the Dev Discussion List again. But then the Back button disappears; I can't use it to navigate back to my previously-viewed Tag or All discussions, or the discussion I was looking at before that. It's not always present nor keeping a 'history', like the browser button is.
I conclude that the button does have a useful and actually typical navigational function, but that the usage of the 'Arrow' icon is causing unnecessary confusion with the browser back button.
Suggestion: Replace the 'Back Button' with the 'Tag' Icon (coloured to represent the Tag you were last viewing) - or the 'All Discussions' Icon (if that's where you were).
Alongside an always-visible 'Pin' icon, I feel that this would communicate better (e.gs):
Go back to Dev, or Pin it in a side panel.
Go back to All Discussions, or Pin it in a side panel.