You just have to try them to really understand. They just seem more stable across the board: in the rough, going fast, cornering. Might be a leverage thing? I don't think the part about opening up the chest really makes a difference. My road bars are 300mm narrower than my mtb bars and I don't notice any difference breathing.

When I started mountain biking 20 years ago, I was running a 135mm stem and 565mm wide bar Over the next 13 years my bar gradually crept up to 750mm and my stem down to 50-60mm. Every time I tried something wider it felt better. Same with all the folks I ride with..I tried wider than 750, but at that point I don't seem to notice an improvement.

Yes, there is theoretically an issue of hitting things with your bars, but I just adapted to it. My hands are at the ends of the bars, so I always know where the end of the bar is. It is really a none issue for me. In the end I don't hit my bars on things any more often. In fact, I do so much less often than when I ran really narrow bars, though that is certainly just due to me being more experienced now..

Hitting stuff or just being too wide on the trail was always my concern when going wider, but it just never really turned out to be an issue.Maybe here and there is a tight squeeze between trees I need to slow down for, but realistically I would have to slow down with 660mm bars as well... maybe just not quite as much. But it is so worth it for the benefits the rest of the time.

There are some other geometry issues as well. Top tubes have been getting longer (accompanied by shorter stems) to push the front wheel out farther in front of the rider (less endo prone). With a short stem (which makes the steering more sensitive) you often want a wider bar to slow it down. A 580mm bar steers fine with a 120mm stem. But swap that for a 50mm stem, and it may seem a but twitchy in the rough.