The app keeps a limited number of messages fresh and up to date, being limited in traffic and data storage.
The default is 25 (same as K-9 or the built-in mail app), but can be changed globally in the app's settings or an account's Options and Folders.
Yes, a filter, or a setting to "keep all starred messages up to date" may come later.
As for the usefulness -- I use it to keep track of which messages need a response, within the recent ones.