When using the App Store search function, the interface operates based on an assumption: that you're seeking apps you haven't yet installed. This presumption guides the display of previews, favoring showcasing app features for those not currently on your device. This means that when you search for a keyword, the previews provided typically belong to apps you haven't installed. The rationale behind this design is to help users make informed decisions about acquiring new apps by offering glimpses of their functionalities.
However, the behavior changes when you tap on an already installed app. In this scenario, the focus shifts. Since you've already installed the app, the assumption is that you're more interested in revisiting its features rather than exploring it for the first time. Consequently, the preview is displayed when you drill down into the installed app, enabling you to quickly access information about its functionalities and updates.
So, to recap, the App Store optimizes the display of previews based on whether the app is already installed or not. For installed apps, the preview is accessible upon selection, assuming you're seeking a closer look at its features. On the other hand, for apps not yet installed, previews are readily available during keyword searches to aid in decision-making about acquiring new apps. This approach aims to streamline user experiences by tailoring previews to suit different user intentions - whether exploring new apps or reevaluating installed ones.