In both the comics and the show, Michonne first appears with two walkers that she essentially keeps as pets.
The comics sort of half-explain this - as per the wiki:
She notices walkers don't attack one another and hacks off the arms and lower jaws of Mike and Terry. She chains them up by their necks to use them as escorts for an escape that was successful.
In S01E02 - Guts, we see that even if they still sort of smell like Walkers, the Walkers can smell/detect the living and will attack.
We see Andrea effectively making a new "pet" so that she can use to deter the Walkers that she comes across on her trip to the prison. Along the trip, she is shown to come within close range of Walkers, but they give her space when she points the "pets" in the right direction.
Why would the Walkers not just attempt to go around the "pets"? What about them creates an effective protective barrier?