It's just a rule from those particular communities. You have to post something before you leave and it has to have the word rule in the title. It's just a silly thing they do.

Getting a bit more in depth, it specifically stems from the r/195 subreddit, which had only one rule stating that if you visit the subreddit page you must post before you leave. This rule was carried over to derivative subreddits like r/196 after 195 shut down. When 196 came to lemmy, this style of post carried over.

It's a meme rule

Just block the subs that do it and never waste another second of your life thinking about it.

Le epic redditors i'd presume