Nearly every time I travel I'm working on a PowerPoint or some kind of documentation for an hour or two in the airport, then two hours on the plane, finishing it up at a coffee shop when I meet up with my coworkers, then presenting it for several hours at the client. None of those places are guaranteed to have power available to me.
Sure a lot of people have laptops that sit at their desk always plugged into an outlet, only running on battery when they pop into a meeting for an hour or two, but plenty more people actually need portable devices. That's the entire reason the MacBook Air was such a smash hit, you see them everywhere with both traveling businessmen and college kids, both of whom need something light and portable with amazing battery life. Both of whom are willing to forego an octo-core i7 and 32gb of RAM in exchange for 10 hours of battery life in a device that weighs less than 3 lbs.
Luckily difference devices exist for the different use cases that different people have.
Yes, almost literally every day.
My typical schedule is to show up for standup at 9, then be at a diner by 9:30. I work from there until noon, then back to the office until 2 to charge my laptop. At 2 I leave and work from a hammock in a park or something similar.
That's ~6 hours every day of working where there isn't a power outlet nearby.
Hell, even the Apple browser is more respectful of battery life than its competitors. Power draw seems to be considered carefully at every level at Apple, and it shows in their results.