Drone Patrol at Bottom of Ramp?

So I was watching this replay on youtube of StarCraft 2 - SC443 - Argos (Z) vs DreamFou (P) on Metalopolis Part 1. Right at 1:52-1:53 mark Argos sends one of his drones to do a patrol at the bottom of his main ramp. The commentator mentions that it is to keep DreamPhou's probe from building a pylon.

Is the point for the pylon to keep Zerg from coming out to mine and/or defend his expansion?

Here is a link to that exact time spot
http://www.youtube.com/watch?feature=player_detailpage&v=pyCUAiyWWWw#t=114s

 

It's more to delay it. But it's also somewhat useful when the hatchery is already growing.

 

Its a preventative move to stop a opponent from doing a wall in. A wall in when you fast expand isolates your expansion and pretty much you gotta cancel it. if its up your gonna lose it. Though i get annoyed when a terran halts and Ebay at my natural to prevent a FE.