I think of a miter box more as a carpenter's tool, but I think were it really shines repeating a lot of pieces of the same length, and making accurate angles (whether or 90 degrees or otherwise) without marking; I think a lot of us could probably make the majority of the cuts a miter box does without one, and to that level of accuracy, but I would need to mark them first to cut to level of accuracy. I can get pretty close to 90 without marking across the stock if things are narrow, (and being narrow, just shooting the edge is pretty quick to fix any out of square) but grabbing a square and a knife or pencil is needed for anything wider. If you're doing a lot of sawing, being able to remove that marking step and just work off a tick mark could make things faster. If you're marking parts off of each other or working in another fashion, it's less of a time saver. Just my opinion, I s'pose.
" Be willing to make mistakes in your basements, garages, apartments and palaces. I have made many. Your first attempts may be poor. They will not be futile. " - M.S. Bickford, Mouldings In Practice