It make you produce a stress hormone called Cortisol, which in turn break down muscle and make you gain fat. Also it usually pound the joints to dust since your muscles are too loose to absorb the impact. all in all, surely an inferior practice.
In HIIT on the other hand your muscles absorb the shocks of the movement and take away unnecessary and floppy load off of your joints. Hence, muscles get stronger. The body secrete androgen which are helpful hormones that help build muscle and burn fat in addition for increasing libido (so that after your workout you are in a more positive social mood).
Heart rate go higher then in the low intensity version, which improve cardiac vascular systems to a much better extent. And you can instal better patterns of movement that can help with other skill work, since the many repetition performed are that much more controlled.
For further information and comparison between the two, click here:"http://www.simplyshredded.com/cardio-for-fat-loss-high-intensity-interval-training-cardio-vs-low-intensity-steady-state-cardio.html