In some cases, as a one-off event, after installing an update to Astro Gold, or if restoring it from a backup or from iCloud, you may be prompted to enter your iTunes password. The reason for this is that, starting with iOS7, Apple implemented a system whereby they automatically store your app’s purchase receipt, encrypted, within the app itself, at the time of purchase. They recommend that any apps of significant value (and Astro Gold is certainly one of those) look for this embedded encrypted receipt to ensure that the app is not a jailbroken/pirated version, and if not present then, automatically, to request verification from Apple. If this receipt is not present, the app asks Apple to query their records for your iTunes account, for which they prompt you for your password, and when they verify that you did indeed purchase it, they insert the encrypted receipt into the app, so that in future it is always recognised as a bone-fide installation.

Note that this is NOT a prompt to purchase anything, and it is a prompt issued by Apple, and not by the app. Note that if it was for a purchase, Apple would ALWAYS tell you exactly what you are about to purchase, how much and ask your explicit permission before they charge your account, and in this case that does not happen. Instead they just require your password in order to verify that you are the owner who purchased the app, and thereafter this will not be required again.