My guess is just a design. Most all the houses I have lived in have had fireplaces, and none of them, even in this 1820s house (where cooking was done in the fireplaces) had any contraptions over the firebox.

I think it is just part of the design. You might use it to display an art tile, or a motto.

Structurally there is no space for a box at that point in the firebox, the smoke shelf, or the flue which are directly behind that indentation.
It is part of the design - a bit of playing with the brick in an Arts and Crafts 'celebrate the material' kind of way. It could have had art tile in it, or perhaps something like a copper bas-relief sculptural panel.
Look for 'Bungalow', 'Prairie' or 'Mission' fireplaces as well as 'Arts and Crafts' for comparisons.