I have a simple answer for that: traffic.

In primary flight training, we're all taught to watch out for other airplanes: "See and avoid." That's one of the keys to safe flying. In good weather---the only kind you're allowed to fly in during primary training---that means looking outside for other airplanes. As we move on from primary training and start flying in less-than-perfect weather conditions, it becomes more difficult to see other traffic. On cross-country flights, most pilots ask for "flight following," so that ATC will monitor your flight and call out traffic. That system has been used for decades, and most of the time it works pretty well.

It's important to remember, though, that ATC's primary job is to provide separation services to aircraft on IFR flight plans; "See and avoid," really isn't an option in clouds. If you fly in congested areas, like Los Angeles or New York, you may find that controllers won't be able to provide flight following---they're simply too busy calling out traffic to IFR flights.

Suppose there was a way to have traffic called out for you without ATC's help?

Pilots of airliners, executive jets, and some particularly well-equipped piston singles and twins, have that today---using traffic alert and collision avoidance system (TCAS), which electronically interrogates transponders on nearby airplanes and displays their position on a cockpit display. TCAS also automatically calculates the course of nearby targets, and in case of a conflict, provides an audible alert. Those alerts have saved many lives.

Unfortunately, TCAS is expensive---typical installations cost tens of thousands of dollars. Even if it were cheap, it wouldn't work if every airplane in congested airspace used it. At any given time in the Los Angeles area, there may be from several dozen to well over 100 airplanes operating. If all of them tried to interrogate each other's transponder, nobody would get any useful reports.

That's where ADS-B comes in: Instead of interrogating transponders, each airplane continuously transmits its position, course, speed, N-number and other information. Other airplanes nearby can receive those position reports and do what TCAS does---but without interrogating each other's transponders. Ground stations receive those reports and provide them to air traffic controllers, and also relay reports between airplanes using different kinds of ADS-B systems (for technical reasons, there are two).

Many of us already benefit from this system to some extent---several companies sell portable ADS-B receivers, which display traffic and weather on portable devices. I use one myself. Having live weather information in the cockpit is a huge help on cross-country flights, and that's available to anyone with a working ADS-B receiver.

Here's the catch: The receiver only shows traffic from other aircraft equipped with ADS-B Out transmitters (and in some cases, Mode-S transponders). Most of my flying is in central California. My receiver frequently shows high-altitude traffic, mainly heavy jets well above me. It rarely shows any low-altitude traffic because most people don't have ADS-B transmitters.

Several months ago, I was able to fly with an uncertified portable ADS-B Out trans­mitter. With that turned on, my portable receiver started showing lots of traffic---because nearby ADS-B ground stations knew where I was and relayed traffic reports to me generated from ATC radar. It was an eye-opening experience: The system pointed out quite a bit of low-altitude traffic that I wasn't seeing with the Mark-I eyeball.

There are lots of other reasons why ADS-B Out is a good idea, especially for instrument pilots. It's more accurate than radar. Radars fail, and when they do, it can be impossible to get an ATC clearance on routes (that has happened to me in the Los Angeles area). Once ADS-B is fully deployed, radar outages will be a nonevent.

On the other hand, I'm sure that ADS-B ground systems will fail from time to time. The FAA is planning to maintain approx­imately half of the existing radars as a backup, which is why you still need a Mode-C transponder, as well as an ADS-B trans­mitter (or an ADS-B compatible Mode-S transponder, which can do both jobs).

ADS-B may also be of critical importance as unmanned aerial vehicles (UAVs) become common. I'm not worried about two-pound quadcopters (provided they keep away from airports), but the time is coming where you'll see relatively large UAVs used for things like crop-dusting and fire fighting. ADS-B will allow these flying robots to "see" us electronically and also for us to "see" them.

To make these things work requires every airplane to be equipped with an ADS-B transmitter, and that's why the FAA has published a mandatory equipage requirement. If you want to fly in controlled airspace after January 1, 2020, you'll need a working ADS-B transmitter. There are exceptions (mainly for airplanes that don't have an onboard electrical system), but most of us are going to have to be equipped.

In the near term though, from my perspective as a pilot, the best reason to get an ADS-B Out transmitter is getting traffic on my portable device that shows airplanes near me. That's why I'm talking to my airplane partners about an ADS-B upgrade at our next annual inspection.