It's easier on your body
During a marathon, runners often experience something that's been dubbed "hitting the wall" (already sounds spectacular), which could also be described as "the moment you realize the first person to do one of these fell down dead."
Hitting the wall is when most of your body's glycogen stores -- your main source of energy -- have been depleted. Your body starts to break down fat and protein for energy, but this takes longer. Your blood sugar is low, and you can start to feel dizzy, shaky, and confused. This tends to happen for runners around mile 20, with another 6.2 miles to go. That's a full 10K, and your body's out of energy.
Screw that! In a half-marathon, you never even get to that 20-mile mark. Catastrophe averted. That's not to say that a half is easy peasy -- you still need to prepare for this stuff, because you risk ankle sprains, muscle strains, dehydration, overhydration, and loads of other ways that your body can bug out on you if you don't know what you're doing or don't have the proper medical clearance. But the destruction that can befall even healthy, prepared people during the latter half of the marathon just doesn't happen, because that's a hell of a lot of miles, and your body wasn't designed to run them.
It's an excuse to travel
It would be ridiculous to fly hundreds of miles across the country to participate in the Brooklyn 5K, but it's not as crazy to come from far away for something like the Brooklyn Half, for example, which involves a seven-day pre-party with local artists and food vendors. “There’s entertainment all day long and into the night, and it’s very much a collective of the community,” Ciaccia describes. “It gives you the whole flavor of Brooklyn.”
After the week-long throw-down you get to shake off your hangover and put on your sneakersYou do it for an opportunity to get to know an appreciable 13.1 miles of a pretty cool borough, and to run alongside more than 25,000 people in what in 2015 was the largest half-marathon in the country. Even better, you're not totally wrecked after completing half of a real marathon, so you can actually spend time enjoying wherever your race was held.
The 13.1 mile distance is also an excellent one for dressing up for a themed race -- long enough to flaunt your attire, but short enough that you may not feel the need to tear off your Jedi costume because you've sweated through it. Runner's World publishes a list of US destination marathons that feature breathtaking views, Star Wars characters, pirates, and more. Try that with a full marathon, and you're simply a novelty character, a fun distraction among a crowd of serious runners.
It's a great look for social media
All of the above combine to create the perfect storm for social media: the veneer of a perfect life filled with health, travel to new cities, outdoor scenery captured mid-run, all capped off by brunch. Just look at all these people talking about their #halfmarathon! More than a million of them, in all their perfectly curated glory.
Of course, completing a half-marathon isn't a bad thing. If that's what gets you out and moving, more power to you. Just, you know, take it easy on the social media posts.