I can't see how there would be a correlation between answering this question well and being a good Product Manager.
What if there's a product planned that specifically aims at piano tuners? Or a feature change that would affect piano tuners? What if Google Maps wants to add a "Find your local piano tuner" mode? Estimating how many potential users/customers there are, how to reach them and whether it's a significant segment is pretty useful. Sure, the example is bunk but it's meant to be something you have no clue about off the top of your head.
What if there's a product planned that specifically aims at piano tuners? Or a feature change that would affect piano tuners? What if Google Maps wants to add a "Find your local piano tuner" mode? Estimating how many potential users/customers there are, how to reach them and whether it's a significant segment is pretty useful. Sure, the example is bunk but it's meant to be something you have no clue about off the top of your head.