Got a mini-lathe for Christmas last year and needed something to put it on. Hundreds of spinning tops and mushrooms later, I’m pretty sure it was worth it. Here’s a couple other things from my lathe.
Ease of construction is the answer. The lower shelf rests all of its weight on the shorter leg portion 2x4 making a kind of faux half lap joint. If they used one chunk of wood for the leg instead of three pieces they would have to remove the material where the shelf nests into the leg which would take more time and complexity to the work.