Because it is important to the myths of each religion. Look at all the cosmological structures in the real world. Several for different religions. And even physicists have disputes.A reasonable argument can be made for "why even have it if it never matters"?
Let's say that based on theory 1, every thousand years on Halloween all gates redirect to the Shadowfell, and that this event is supposed to take place 100 days into the start of the campaign. 100 days in, the PCs are in a bad spot, stuck between a portal into the heart of the Plane of Elemental Fire and an enemy they can't hope to defeat. One of the players suddenly remembers this throwaway tidbit, and tells everyone to jump into the portal. I think it's fairly pertinent to know whether they're instantly incinerated or transported to the Shadowfell, don't you?
I'm not saying there's anything wrong with deciding on the spot, but if you decided it works this way ahead of time you can include little clues to that fact throughout the campaign.