The absence of a rippled or heavily textured surface changes the way Underground behaves on a wall. There is less competing with the colour, the grout or the surrounding finishes, so the overall result feels more deliberate and architectural.
That makes Underground useful when the rest of the room already has plenty going on. Detailed cabinetry, patterned flooring, strong worktops or interesting fittings can sit alongside it without the wall becoming another source of visual noise.
It can also be a good choice if you're aiming for a very clean tiled backdrop. The flat face and straightforward proportions create a consistent surface, allowing the colour to carry more of the design.