Wrapping sandwiches, deli-style
I've seen the method described above work well done on a diagonal as well, with the sandwich lined up roughly corner to corner (a bit closer to the first flap folded than the last since you generally want a bit more slack on the last flap to close it up nicely) you fold one corner over the sandwich then fold in the two adjacent corners and roll/flip the sandwich to fold in the final corner.
This is particularly handy with things like subs since you can use a shorter rectangular sheet of paper with better results, just start with a corner along one of the long sides (away from the sub ends) and roll/fold it over the sub, fold in the corners at the ends and roll the remaining flap/corner around the sub.

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