Nutrition and normal behaviour
Feeding horses well involves more than just supplying all the necessary nutrients. Given the choice, horses spend about 70% of their time looking for food and eating; therefore food and eating plays a significant role in the day-to-day behaviour patterns of every horse. Horses have evolved to survive on low quality herbage eaten in large quantities, which is why they are innately driven to eat for almost three quarters of their time. Given their natural diet, they have to spend this long in order to obtain enough nutrients!
Research has shown that feeding horses with no regard to their innate behaviour results in a myriad of problems, from stomach (gastric) ulcers to abnormal behaviour such as crib-biting, the development of growth problems and colic and difficult, reactive behaviour.
If you take into account the innate (normal) behaviour needs of horses, and feed and manage them accordingly, you will have a healthier, happier horse who has better welfare.
What does this mean practically? It means getting as many nutrients as possible from forage and adding other feeds in the bucket only just to balance what is missing from the forage. Forage takes much longer to eat and needs a lot more chewing than other types of feed. You need to source forage that you can feed plenty of, so for example a low energy (calorie) forage for good doers or overweight horses. For horses with higher requirements e.g. poor doers, working horses or breeding stock, you need to source higher energy and protein forage so that you don't have to feed them high levels of concentrate feed.
Now that's not to say that concentrate feed is bad or unhealthy, and most horses can deal with concentrate fed sensibly, but if you think of your horse's diet as being forage plus whatever balances that forage, your horse will likely be healthier for it (forage does not supply all the essential nutrients a horse needs, so even if they are maintaining weight and condition, you will need to add other feed/supplements for a balanced diet).
By basing your horse's diet around fibrous forage, not only will you be giving them nutrients they've evolved to thrive from, you will give them the opportunity to spend more time eating and chewing, which will benefit their physical and psychological health, and reduce the risk of a myriad of health problems.