The mixing has little to do with how the yeast works, as there is plenty of evidence to show a settled layer will get consumed in time.
Its more about fermentation management and consistency of product. You can't get an accurate reading from a must if its not the same all the way through, without readings, you can only guess at possible strength, if you don't know the strength or likely strength, its hard to diagnose any fault the batch might have, etc etc.
Hence proper mixing, to remove a variable.... just like taking a reading with X about of honey mixed into Y amount of water. Differing honey will give different reading with different sugars/water content, yaddah yaddah......