I ran my marathon PB of 2:52 back in 2006. After a couple of injury-plagued attempts at trying to PR in my 40s, I realized that it'd be very hard for me to get faster. So needing a new challenge, I decided to try to go longer. Since 2017, I've done a 50-miler, a 100K and a self-organized 100 miler last year. The 100K was a trail race and my least favorite ultra because of all the elevation which forced me to walk. I still enjoyed the experience (finished in ~12 hours), but it wasn't my favorite because I like to run rather than walk/hike. I ran all of the 50-miler (under 7 hours), but after 91 miles of running, I was force to walk the last 9 miles of the 100-miler (finishing the flat trail course in ~19:30). No nutrition or stomach problems--the muscles in my legs that are responsible for running just wouldn't work anymore.
All that to say that for people who like to set goals (e.g. sub-3 marathon), ultras can provide another type of challenge and offer a different sense of accomplishment. (That's why I don't think 50K should be counted as an ultra because it's too much like a marathon.) I say "can" provide, because people can finish any distance (5K to 100-miler) but be largely empty of a sense of accomplishment because they didn't put in the training to achieve the best result possible (whether that ends up being a relatively slow time or fast time is to me irrelevant). They just want the experience and all the accompanying photos--which is fine since everyone is different, but to me an ultra as an athletic endeavor, as a longer-distance cousin to the marathon, should mean training for it as seriously as for any other distance.
Which leads to me say that I'm done with ultras. I've enjoyed challenging myself with longer distances, but I realize that there are three main disadvantages that will keep me from attempting any more:
1) It takes way too much time to train. Too many weekends where I was away from family while doing 30, 40, 50-mile training runs. Too many early weekday morning training runs. And since I no longer want to put in the training necessary to be able to run as many of the miles as possible as fast as possible, I have no desire to pay hundreds of dollars to sign up for a 100-miler where I walk a big chunk of it simply to finish (though I can admire the toughness and pain-tolerance of those who finish 100-milers in 24-30 hours).
2) At age 50+, all those training miles (even done at ~8-9 minute/miles) take a toll on my body, making me less energetic and productive for all the other areas of my life.
3) My legs have never been in such pain as the hours and days after the 100-miler. (In comparison, I was able to recover fairly easily and without much pain after the 50-miler and 100K.) Simply put, I never want to experience such pain again.
Instead, I'm thinking of turning to 5Ks and 10Ks as the next challenge. I've only run one race under marathon distance (a 5K in 1997), so it'd be a new training and racing challenge for me, one that'd require fewer hours of training.