I often switch between working on my desktop and my laptop. keeping to a limit of about 80 characters makes it easier when working on the laptop. And as others have said, I often split screens when working on my desktop as well, so it helps there too.

I also find that very long lines can be a code smell - deeply nested callbacks or a case of 'divitis' in html markup.

When working with html, I find it convenient to put attributes on separate lines if there are more than a couple of short ones. This naturally helps keep line length down. Having them on separate lines helps with editing a bit too.