Alaska requires that all vehicles travelling more than 45 miles per hour must have their lights on. Most states require that lights be on at least a half hour before sunset and a half hour after sunrise as well as during any precipitation. Yet today, while driving through snowy weather that occasionally turned to white-out conditions, I saw numerous cars travelling without their lights on.
It isn't about what you can see; it's about other people seeing you. Even on clear days it can be difficult to see other vehicles on the road if their lights aren't on (I find this particularly true with grey vehicles that seem to just blend in with the road). Many newer cars, such as my Prius, are now equipped with daytime running lights specifically for the purpose of making accidents less likely.
It is one more thing to remember after starting the car (and I even forget from time to time), but it's a simple step that makes one of the most hazardous tasks that we do on a regular basis just a little bit safer. Drive with your lights on. The life you save might be your own.