Adding to the other answers, there are also -besides Photoshop, Gimp, etc- specialized tools to draw fantasy maps. The site mentions Wonderdraft [0], but there are a bunch of others though not all of them support using external brushes.
Some other tools in this space may be Watabou's tools [1], Azgaar's tools [2], Inkarnate [3], Mapforge [4], or quite a few more which you can find links to in this list [5]. Again: you could use these brushes with some of these; not all support external brushes.
I have not looked at these specifically, but can give a simple example.
You're probably familiar with how the maps look in Lord of the Rings.
If you notice, the mountains have a look to them, a schematic representation of essentially triangles, perhaps done with a fine brush. It's not just two straight lines in a corner.
So, anyway, one of the brushes in these sets might be those mountain icons.
Then, when making your mountain range on your map, you select the "mountain top brush", and then "stamp" mountains in an area using the glyph. So, in that sense, it's not a "brush" per se, something that you actually use for brush strokes. Rather, its an iconography that you can place on your map.
That's a trivial example of how these can be used. They may well be more sophisticated than that.
In traditional painting, since hundreds or thousands of years ago, many drawing techniques relied heavily upon using different physical brushes. The properties of each different brush would make certain tasks easier for the painter.
In more modern times, as computer-based drawing evolved, that same principle became a basic feature of drawing applications - that is, the ability to change between different (virtual) 'brushes', each with different characteristics, such as shape, but also intensity and other factors which make the artist's job both easier and more nuanced.
In a way, your reduction of the idea to "a set of rubber stamps of sorts" is true, in the same way that a physical brush is equally "a stamp of sorts", if the artist chooses to use that brush in a suitably monotonous stamping action. But in digital art (as in physical art) the total picture is more about the skilful wielding of myriad tools and techniques, through multiple steps and recombinations.
To bring in another analogy - every wood worker uses basically the same tools, the basic claw hammer shape hasn't changed in a long time, the basic screw drivers, drill bits, etc. The same set of rubber stamps, in a way. Is that a constraint upon creativity and uniqueness? Perhaps, but not meaningfully.
