Good boxing training will teach you proper foot work, get you used to taking punches (which is a key part of self defense), and obviously teach you how to punch properly and greatly increase your speed. Many people are not flexible or fast enough to properly throw kicks, particularly in a real world scenario, so all of the time spent learning fancy kicks in TKD and to a lesser degree Karate are basically useless for self defense. Boxing is basic and effective. Most fights start with and can be ended with a punch or elbow. The vast majority of people who aren't trained don't know how to throw a proper tight punch with proper body rotation and use of your legs and core that can generate tremendous force.
I saw on "Human Weapon" or another similar show that they tested punch strength of TKD, kung fu and boxer, and the boxer far and away had the most force behind his punch.