Sacred Haven of Debauchery

One of the systems they are suggesting is completely useless to me.

You can make them a vassal. Or give it to someone else. Don't forget about this option; you can even use it to make the Fallen Empire angry at your future target if their sacred world/planet is in close proximity to the borders.

Added after 47 seconds

Hmm. I'm having a strange game. It's endgame already, but throughout the entire game, no one has declared war on anyone. The galaxy has long been divided, and it's unclear what the competitors are doing. Maybe it's a bug?

If you're playing version 1.6.0, then yes, you have a bug. In this version, the AI doesn't declare war at all.