Aesthetics, and having a proper retaining wall should this world ever suddenly decide to start raining. The garden wouldn't last long should the soil get washed away!

"Without means to cut or shape the stone, some unevenness can't be helped," Merlin points out. "But if it's too low on the lower side of the slope, then if it ever rains, there's a chance of the soil being swept away."

That bit of knowledge shared, Merlin watches for a bit as the robot gets to work. He might help if asked, but if not, well, he sees no reason to step in on someone who's learning through practice.