You'll see bigger results without spending more time on your workout: If you previously walked 30 minutes a day at a steady pace, you don't have to increase the duration of your workout to see a bigger payoff, because you are just increasing the intensity.
It's better for your health: Interval training is widely used to increase cardiovascular fitness and therefore health. Any activity that elevates your heart rate and requires increased physical effort helps to strengthen your heart.
It'll decrease your body fat: During the bouts of higher intensity you are burning more calories than during steady state walking. A higher caloric deficit means you will burn more fat and lose more pounds and inches in a shorter amount of time.