Steady-state cardio is any type of aerobic exercise that is performed at a low intensity for a prolonged period of time. Typically, it is done anywhere between 20 minutes to an hour.
If cardio is THE ONLY form of exercise that you do, then it is very important for overall health.
But if you do other specific exercises, it becomes less important...
Performing low-intensity steady-state cardio can and will make the heart more efficient, improving heart health. But these benefits aren't exclusive to cardio...
The thought process is that running will burn a lot of calories, and therefore you will be in a calorie deficit.
The truth of the matter is, you cannot outrun a bad diet.