You are notified when they publish new hubs or when comments are made on any of their Hubs you choose to follow. I follow over a 100 writers and that allows me to pick and choose which Hubs I maybe interested in reading.
It is mostly a convenience thing but it also allows you to easily interact with writers that publish on similar interest and then jump right to their latest publications, questions and forums.
for instance if you were to follow me then you would have access to all the latest information on the ongoing Polar Shift, which many are unaware of . . .