Instances can grow fast, and that can be a problem if you have limited disk space. If you don't mind losing history that your users have not interacted with, LPP can help. It will purge posts/media older than a specified time if they have not been posted, marked read, voted on, commented on, had comments voted on, or saved by users on your instance.
Pict-rs processing is optional. If there is concern that Lemmy is not purging from Pict-rs, or that Pict-rs is not removing files from the file system, then this extra processing will ensure that occurs. Supplying PICTRS_RM_OLDER_THAN_DAYS along with a URL and server admin token will run a purge command directly against Pict-rs for media that falls within the above criteria of no users on your instance interacting with them. In addition, supplying a PICTRS_FOLDER will also scan the folder for images that were not properly removed from the file system during the purge and force remove them.
Latest documentation available at: https://nowsci.com/lemmy/lpp/