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 can help you lose weight: During the higher-intensity bouts you are burning more calories than during steady-state walking. A higher calorie burn means it can potentially help you lose weight.