These days, a headline like this sounds like some kind of radical statement in the equestrian world. Because of the prevalence of show barns and horses raised for sporting, there is a great number of horses that spend long amounts of time cooped up inside stalls. And why wouldn't they? Ever since the horse became domesticated, humans have strived to find ever-more efficient and easy ways to keep horses. Stalls make it easy to feed, easy to keep horses clean and scar-free and un-faded for shows, and easy to fetch your mount when it's time to ride.
The only party that stalls aren't so good for, however, is the horses themselves. Horses are naturally designed grazers, meant to spend a majority of their day walking from place to place while eating. That fact has never changed throughout the centuries.
Turnout - at least a few hours per day to roam free from a stall, preferably on grass - is incredibly important to the physical, social and mental health of horses. And we've seen the evidence of what can happen when horses don't get enough turnout: stable vices (cribbing, weaving, box-walking, etc.), problems with gut health (colic), other stereotyped behaviors (head bobbing, for example). These behaviors are rarely present in horses that are on pasture for at least a majority of the day, and yet are frequently observed in horses that spend long amounts of time in a stall.
As with most things with horses, the important question to ask ourselves is not "what do I need from my horse's living situation?", but "what is it that my horse needs?". When we consider the horse's ethology, it becomes very clear: there is a myriad of benefits to keeping a horse on pasture, because that is how they are designed to live. It's just a matter of shifting our own perspective to consider the horse's needs above our own.