Typically that's where Haley sits, who is typically the person you'll first meet when you're visiting or if you're dealing with some logistics with the SF office.
(As an aside, the idea behind the Oval Office as a visitor's reception is to turn the notion on its head a little bit. We've all been to companies where the reception area is just an afterthought, and you're stuck awkwardly standing in the middle of an open floor, not knowing what to do next. Our Oval is a tongue-in-cheek jab at that, and an attempt to make things a little more, well, interesting while you're waiting for a GitHubber to meet you.)
To add to what Holman said, at most companies the receptionist is considered unimportant, and is treated as such.
GitHub turns that on its head. Haley sits at the president's desk and is entrusted with people's first experience with GitHub's HQ. She's an important part of our culture. It's an big responsibility and a position deserving respect (as all positions are).
1. http://thenextweb.com/shareables/2013/09/07/githubs-new-offi...