Some products are like parasites: the product benefits the developer at the expense of the host. A highly addictive mobile game like Candy Crush might fit the criteria.
A mutualist relationship is one in which organisms from two species exist in a relationship in which each benefits from the other. You might have this kind of relationship with your gym.
If you’re a builder of a mobile app, how might you ensure that your product is in a mutualist relationship with its users?