What I believe the real answer is, is that the restaurant is likely trying to beat the JIT (Just In Time) production system. For example: If you are a restaurant themed around southern comfort, some of that food can take quite a while to prepare. To get ahead of the curve, you avoid making food in response to each order, and instead opt to make food in bulk at several points along the day, knowing you get X orders on average every few hours. For a sandwich joint, it probably has some additional nuance to it since its not quite as time intensive to make one, but I believe this to be the general answer.

Edit: this works better for some restaurants than others. It brings a higher production cost to cover the waste that went unused, but cuts out costs for labor. And for some joints like a Taco Bell, it can be optimized well enough that it'll hardly make a difference whether they make it on the fly, or in advance.