Why is it profitable to goof off?

Last Thursday my desktop was running the tests, I had read all my mail so there was nothing to do on my notebook, so I decided to grab a book I had to read for the job , a Diet Coke and go to the massage chair (OK, it was not goofing off, but the title makes a bigger hit this way). In other companies they would tell me that if I want to read, I do it in my own time, and of course, pay for the book myself.
Maybe that was not the most profitable hour that Google paid, but it meant that on Friday I really wanted to get a test suite to run in 60 seconds, no matter if I had to stay until 2 AM (which I did).