The most well-established benefit of HIIT...

...has to do with heart health: Intervals can boost cardio-respiratory health with a smaller time investment compared to continuous forms of exercise. 

It's not about superior fat-burning capacity or bigger muscles, but about improved VO2 max, a measure of endurance that calculates the maximum volume of oxygen the body can use.

VO2 max is one of the best predictors of overall health.