A low mode. Many front lights have it but most rear lights don't. If you're riding at night in a pace line it's really annoying and possibly dangerous when the guy in front of you has his blindingly bright rear light aimed in your eyes. We require people to put their lights on non-blinking modes but some are still far too blinding especially if they got bumped, are aimed too high, or are clipped to a saddle bag. For the front light a low mode is important if you're behind a guy with a wimpy light. Try taking a pull with a wimpy light when the guy behind you has a 1000 lumen monster light. You can see to your right and left but everything directly in front of you is a dark shadow.