Like many of the other blogs suggest, I believe it was for reassurance. The diciples were unaware, yet they carried out the instructions issued by the bystander on the beach. Why did they listen? Perhaps, based on their relationship with the master, it fostered a trust and a willingness to open themselves to such situations.  In this instance, perhaps JESUS was trying to teach them that the master can appear in many different forms, and more importantly would always there to support & guide them.