See sometimes just being alone isn’t enough. You can work from home or put headphones on and still be distracted by the online world.
Going offline gets rid of those distractions. No email. No chat. No RSS. No web. That leaves you with only tasks. Things that you need to do. Action you need to take.
See, there’s an inherent problem with always being online: you’re too connected. You wind up in the role of passive observer. Things come to you. You react instead of act. You can easily spend too much time “marking things as read,” reading RSS feeds, watching YouTube clips, or whatever else.
When you go offline, that equation changes. You have to be active. Since you can’t input, you output. If you don’t do something, nothing happens.
So turn AirPort off. Or go to a coffeeshop without wifi. Resist the siren song of being connected (for a couple of hours at least) and watch your productivity skyrocket.