With retention, you can answer questions like "After users install our mobile game, how often do they come back and start a new game?" You can split your users into cohorts based on when they complete an action (in this case, the "Install App" event). This lets you visualize how engagement and time-to-activation change over time.
You can also see how retention changes for different user types. Are you better retaining users from a certain country or referral source? Marketing teams can determine which ad campaigns lead to the most loyal users. Product managers can measure how retention changes across mobile and web form factors.