I may be in the minority but I find this feature quite useful. As someone whose is a more visual learning person, having the ability to virtually see how my product will turn out makes it easier for me to make choices. Instead of thinking about if item X and Y go together, it gives me a better idea of how it may taste when I see them stacked on top of each other. I know X tastes like this, so seeing them next to each other, it may allow me to better think if tastes will work together.
We also can’t forget about people who may not be able to read properly. Instead of worrying what they’re ordering, they just make it look like what they want. (somewhat of an extreme case but possible)
So while I agree this small design is not a high priority for UX designers, I do think not having it would be somewhat of an accessibility barrier for some users.