Some sites don’t need tagline, however, even these sites would benefit from it. No matter how well known you are, why pass up a chance to tell people why they’re better off at your site? Even if a site comes from a strong offline brand, the mission online isn't the same and it’s important to explain the difference.
Don’t make me think
Being an expert in online usability Krug gathered his experience in a wonderful book. This summary of key thoughts will help you to remember most important principles and laws.