Why are tubeless tyres popular in mountain biking? I've noticed that a lot of people use or promote tubeless tyres for mountain biking. Why is this?

It seems a lot of hassle to get a special rim conversion kit and goop to put in the tyre. As far as I can see it still makes a lot of sense to carry a tube in case it all goes wrong out on the trail any way.
Although this is purely anecdotal, I know from my own experience mountain biking that most of my flats were from so-called "pinch flats" (where the tube is pierced by pinching or friction with the tire/rim rather than piercing by an external object). This was highly annoying. After having switched to tubeless some years ago, I've not yet had a single flat. And, obviously, without a tube to get pinched, there's no way to get a "pinch flat".