Ultimately someone is responsible for that machine, I think that role should fall with the sysadmins, if they exist yet.
Github is large enough that they should have in-house IT looking after the desktops/internal network. They might even have two teams, one looking after employees, and one looking after the server infrastructure.
Start-ups grow organically, so there might not be "someone" at first, other than the developer. This quickly turns into a burden when you have 50+ employees running around and warranties start to lapse, there is no current inventory list, you have a mish-mash of machine configs. Then comes the stage of getting a consistent config on each machine, so that person X and person Y can collaborate, using the same dev tools, at the same rev level. Looks like Puppet/Boxen is a tool to solve that problem.
This problem is compounded when you are in a hiring frenzy, because new people almost appear out of thin air, and you are supposed to have a machine for them. Having a quick deploy script will save your hair line considerably.
I guess how useful it is for a sysadmin would depend on whether changes are pushed automatically to the macs.