Because I can. Giles has finally bothered to go to 'work' and is sitting in the projection room of the theater. He literally has a guitar with him as the aliens had provided him with it. Giles hadn't at all intended on singing to the public, he's not that type of person any more. At first he's just humming and then finally bursts out into Freebird.
The video cuts when a fire accidentally starts behind him in the projection equipment. The last image is of him breaking into another song about it being a Thankless Job.