We all know the original default structure of the planes. Each an infinite realm laid on top of one another. The inner planes (positive/negative/fire/earth/air/water) are the building blocks of the prime material. Upon the prime material sits the a shadowy ethereal plane, the conduit to the inner planes, and the astral plane which warps distances and provides access to the outer planes - realms of reward and damnation. (or any specific configuration of the above as you desire).
What does this have to do with swords?
"Assume further that creatures which can be harmed only by weapons of a special metal (silver, cold iron, etc.) gain this relative invulnerability from having a portion of their existence in either the positive or negative material plane at the same time they exist partially in the prime. Therefore, those creatures which can be struck only with + 1 or greater magical weapons exist wholly and simultaneously in two planes (one of which is, of course, the Prime Material). So creatures which require attack of a + 2 or better magic weapon then exist in three planes simultaneously, and so on. This brings us to the consideration of the existence of magical weapons in other planes and in multiple planes simultaneously. If it is accepted that the reason that certain creatures can only be hit by magical weaponry is because the creature exists in two or more planes simultaneously, then it follows that the weapon must likewise extend into the planes in which the creature exists. At the very least it must be that the weapon extends into no less than two of the planes in which the creature exists, and these planes are those in which the creature has vulnerable aspects. This makes for a very complex relationship of planes to planes/swords and other magical weapons to planes. [ed. I'll say!]
A special sword functioning with bonuses against certain creatures, or a special purpose sword, will have existence on only certain planes with regard to its special bonus, or due to its special purpose, but as most weapons of this type also have a general + 1 or better value, they also extend into all planes — or do they?" - Gary Gygax, Dragon Magazine July 1977
So here we have codified the thoughts behind +x or greater magic weapon. This doesn't address the actual game intent of monsters that cannot be harmed by magical weapons, but it does provide a chohate reasoning for why certain creatures are only affected by magic weapons.
The value in this is the insight provided as to the reason to map out the planes. This is done so the Dungeon Master can track which weapons exist in which planes so he can determine which monsters are affected by them. (Also justifying the popular 'magic sword +1, +X versus Y).
This is the gestalt of the old school style. A complex logical reasoning, independent to each game that explains how things work. The actual way that they work is mildly irrelevant, just so long as internal consistency is maintained.
And the purpose of creatures that can only be hit by magical weapons? Twofold.
First is the ability of the referee to create an environment that may not be open to the player characters. The inability of players to at one time go everywhere and do everything is a key component of the style - it is important to have pockets of areas where players can't go. (I'm doing this currently in my weekly game. I'm surprised one of the players isn't keeping a list of places they can't enter.) This is especially important in a mega-dungeon, because it allows you to place shortcuts to lower levels guarded by beasts that lower level characters can't harm. ("You must be this tall to ride".)
Second, it forces the players, in a quite sudden and shocking way, out of the drudgery of "I hit it with my axe". These monsters may not be affected by weapons, but they may suffocate, be trapped, overborne, or have their actions and lives curtailed in a variety of other entertaining methods too creative to list them all here. In short, it forces creativity.
Contrast with TETSNBN model of Damage Reduction working as air brakes. (This was a less of an issue of 3.0, where the values were high enough that no one was breaking through without the appropriate weapon.)
Just a short note, that both the treasure and psionic supplement are done. They are in layout and will be up soon.