Such apps helps you to stay in your desired interval training program. Say for the first five minutes, my target heart rate zone is 100-110 and for the second five minutes would be 110-130 and so on. If I go below or above those ranges that I've set on those intervals I will be alerted by the app.
Most of the apps in google play just display your current heart rate monitor, record/track it, and nothing more. I'm not looking for such apps.