One could, but I think part of being a good app developer is understanding how people use the platform. Just like Boeing needs pilots, Ford needs drivers, etc.
Imagine trying to develop a web application if you only used a browser on your own development and production website (ie: that you otherwise "left it on your desk")
Imagine trying to develop a web application if you only used a browser on your own development and production website (ie: that you otherwise "left it on your desk")