Its not a workout, so dont fool yourself.
You sweat becuase sweat is your body's way of regulating bodyheat- its too hot so your body sweats trying to cool off. Sweat is also used as part of the excretory system-used to excrete toxins out through your skin.
The heat is definitely relaxing, and relaxation is good for your health, but I would still commit more of my time to concentrated gym efforts. You will see more of a benefit to your body composition that way.
Also, I heat and moisture is a great breeding ground for bacteria. I know some people go in there to "clear thier lungs out", which may help the bronchial passages, but at what cost? Spreading bacteria and viruses in the air?
No thank you. Just my 2 cents.