If memory serves, and it's been years, originally the portals were to show an image of the boss you were to kill for that day back in WotLK.

His function has changed over the years, and when originally implemented, he gave the daily dungeon quest. So if the daily dungeon was to go to, say, Utgarde Pinnacle… An image of ––

Edit, correction: The quests were to kill specific enemies inside said dungeons, so for UP, it would be Ymirjar Berserkers.

Please Help.
Thanks !
I didn't find the right solution from the internet.
References:
-https://us.battle.net/forums/en/wow/topic/20759376148
- Whiteboard video