Xenon headlights first appeared on cars sold in the U.S. back in the 1990s, and they have steadily gained in popularity since then. While their modern, "blue" light has some aesthetic appeal, xenon headlights also provide at least three important functional benefits.
Compared to traditional halogen headlights, xenon headlights produce up to twice as many lumens per headlight bulb, which means that they emit brighter light than was previously available from halogen headlights, even on low-beam settings. When headlights produce brighter light, the task of night driving becomes significantly easier for most people, especially senior citizens.
Xenon headlights typically last two to three times longer than halogen headlights, because they use a completely different method of generating their light. Conventional headlights conduct electricity through a thin wire (called a filament) that is housed in a chamber containing halogen-enriched gas, producing light in a somewhat similar fashion to traditional incandescent household bulbs. On the other hand, xenon headlights don't use filaments that break over time; instead, they create light by directly conducting a beam of electricity through electrodes in a xenon gas-filled chamber.
In addition to providing a brighter view of the road ahead, xenon headlights can provide other safety benefits. For example, most factory xenon headlight systems include auto-leveling devices to reduce glare--they automatically lower the level of the headlight beam when the vehicle goes over bumps in the road or when it carries a heavy load in the rear. Furthermore, many of the newest systems include adaptive features that automatically pivot the headlight beam as the vehicle goes through corners. By pointing the beam both lower and directly through corners, the xenon headlight can reduce (not eliminate) glare for drivers of oncoming/leading cars, and it also directs light more usefully for its own vehicle.
Despite these benefits, xenon headlights are relatively expensive, and it can also be very difficult/impossible to retrofit them to a car that was originally equipped with halogen headlights. Over time, however, xenon headlights are expected to become more widely available for both new and used vehicles.