Contrary to the instructor who once said there is only "On or Off" for muscle engagement. I really hope I misunderstood their intention with that.

There is a redundency factor in our bodies. If you had one main nerve firing one entire muscle that then gets damaged, you would lose entire control over that muscle and its function. If nerve pathways weren't shared, it would individualize group function (motor units). Think of it like roads in a city. The fascicles allow for micro stabilization within the muscles plane of movement, creating strength/integrity through changing force/velocity. (More data required) Developmentally we stress our bodies with different demand and stimuli, creating a reaction in how our muscles create shape. As we all have a similar bone and joint structure (compared to other animals) we have similar patterns to the base demands that our body develops in response to, so similar fascial presentations develop.

Please strike me down for any inaccuracies or incorrect generalizations. It sounds like you have some solid education behind you already.