Klaus said:
Because there's no other reason to have anything in an adventure game if you can't use it in an adventure with your adventurer PCs who adventure in adventuring locales.
Uh, it's not an "adventure game", it's a
Role Playing Game. Things like the planar cosmology help to flesh out the entire setting, providing flavor for role-playing, not just be another site for adventuring.
The way I see it, the first thing is to create a consistent and believable fantasy setting. This may include places either physical or metaphysical that are not optimum places for the Dungeon Crawl Of The Week. Once that setting is created, then the DM creates adventures in the world that involve the PC's. The player characters exist to adventure in the world, the world does not exist to service the characters.
Is Mage: The Awakening a poor RPG because the Supernal Realms can never be visited (aside from in a dream-state during an Awakening) or adventured in? No, because while they aren't places groups of Mages are going to be stalking around hunting monstrosities, they are places of legend and myth that all mages aspire to.
Is Middle Earth a poor setting for RPG's because the Grey Havens are places very few will ever be able to go and even if you got there it would be a boring place to adventure? No, because the Grey Havens are a place of myth and legend, a resting place for the honored and the mighty, not another place to go stomp some orcs.
Is a quasi-historic game set during the middle-ages a bad setting because Heaven would be a lousy place for adventuring? No, because it exists as a core piece of religious belief for the setting not as a place for PC's to go get some XP's beating up Angels.
Was the Forgotten Realms a bad setting (before they blew it up to make it fit 4e) because it had the Demiplane of Cynosure, which was a place no mortal could enter and only Gods could walk in? It sounded like a pretty lousy place for adventuring, but they certainly described it because it was a place of lore for the setting that sages knew about and religiously-knowledgeable PC's would probably know well. Was it a bad place because it had the Isle of Evermeet, where only Elves were allowed to tread and that rule was backed up with artifact-level force? It was a terrible place for adventure, since most parties couldn't set foot there as a whole party, but it existed as a place told of in song and legend and rumor and myth.
D&D is a roleplaying game, some things in the game (like some planes of existence) are there to help roleplaying, not just to provide yet another backdrop for a combat grid.