The people that need to boost their health and lose fat the most need to establish a cardio-respiratory base. Trying to push an unfit individual to their physical extremes can be dangerous. Furthermore, physical discomfort is not conducive to continuing an activity and therefore long term success.
High intensity training requires rest. Even if you are at a higher physical level, interspersing longer, slower cardio into your routine is advisable.
Interval training does not have to be intense. In fact, you can incorporate intervals with the even the most unfit individuals.