If I boot up in the morning, spend 30 minutes trying to get into something, fail and end up back here on HN, I don't bill any time. If I did the same thing sitting in a cube, I'd get paid for it.
The end result is that the client gets a much better deal by having me off site. Works great for me too, since I can justify billing out at a higher rate that reflects the fact that they get essentially all my productive time for the week, and nothing else. Everybody wins.
As an added bonus, if I really can't get started in a morning, I can bail and go bouldering for the day and not feel guilty about it. The bizarro world salaried version of me would spend that same day sitting in a cubicle secretly playing video games on the clock.