The amount of user growth your app has is the first indicator of the success of your mobile app, so you need to monitor total users and new users closely. The number of users will give the broader team the data they need to understand everything from revenue and budgeting, to hiring.
The return rate is the average number of sessions of one user completes over a given time frame.
As a technical team, these user metrics serve as an improvement indicator and where to focus resources. For example, you might see that the return rate is declining. To improve this rate, you might want to create push notifications incentivizing users to come back.