Commentary Purg XVI 10-15

The simile presents an image of Dante walking behind Virgil with his hand upon the shoulder of his guide, otherwise present to him only as a voice.  The image prepares us for what will happen once Marco Lombardo is his interlocutor: all he (and we) will be aware of is a voice, close to an ideal situation for a poet to contrive in order to gain undeflected attention on behalf of a presentation of moral philosophy.