It occurred to me as I was watching someone make three chicken salad sandwiches today that making a sandwich is much like packing a suitcase. One has a confined space in which specific items are layered in a particular order. The placement of each component is deliberate. If one is trying to run from the law or the repo man, however, one will throw everything together willy nilly. There is no time to consider if the mayo or the mustard is dripping.
When I related my thoughts to a co-worker, he added that a sub (think Philly cheese steaks, meatball sandwiches, and French dip) would be more of a duffel bag and less of a suitcase.
A calzone might be a messenger bag. A burrito would be a poster container, no?