The “–watch” feature is great when you want to see near realtime updates of your site. It’s very convenient to type a bit, save the file, and see the results immediately in your browser. When you’re tweaking a theme, it’s indispensable. It’s a real thrill to make a change to a template and see the browser magically update the page.
There’s a cost, though, if you’re running on battery power. On my MacBook Air, the estimated time with a full charge goes from 12 hours of charge to 5 hours when I run with the “–watch” option and Google Chrome at the same time. That’s still plenty long enough for most flights, but if you’re in a crunch, consider running the server without “–watch” and restarting it when you want to see your updates.