Here’s what happened when a developer named Stuart Hall made an app and put it in the app store without any form of marketing. It’s an experiment to find out how the app will perform when you have no credibility as a developer or influential people to back you up.
7 minutes workout is a trending topic. The app may not fare as well if it was to be in a saturated niche field with lots of competition. Getting into a market that’s relatively raw with no clear established leader helps.
As with many things, going free helps to gain momentum and gather massive new customers. I think that’s the main reason why many profitable apps are going this route – a free app with in-app-purchase. I don’t necessarily agree with this model, I would rather pay upfront for something and not worry about future purchases.
Having said that, I understand the rationale of why so many developers are implementing this scheme. That’s how the Apple App Store ecosystem works. This is how we as human behaves.