I am not a developer, but I would assume that devs want to know things about who is downloading their app so they can make tweaks. The Android Market Developer console has been updated to show more detailed stats on apps on the market. The update offers all sorts of new usage stats on apps that devs have on the market.
The stats will allow the dev to see who has installed the app and when it was installed. Data on the version of Android the device was running, the country, and language are also offered up. All of the stats are kept private.
The details are only for the developer of the app and allow app makers to tweak things to suit the audience that is most interested in their wares. Developers can access the new details by clicking the link labeled "statistics" in their console.