Instances that are not trying to attract a new audience might want to turn on the new Private Instance setting, which makes everything private. The home page, posts, communities, etc will all require a login to view. Bots and scrapers won't be able to get anything useful from your instance and will stop trying.

Also if a single-user instance is subscribed to mostly NSFW communities, maybe they don't want everyone to be able to see which communities are listed at /communities.

Or it could be used temporarily when a biiig flood of scrapers turn up, then turn it back off when they go away.

Edit: Federation is unaffected by this. For logged-in users everything continues as before.