Baldur's Gate : The Sword Coast Chronicles : NWN2 Persistent World

Material Metal ( Iron)what is the point if it dosent give any thing

It gives dwarves something to do besides brewing and drinking ale ... one trick ponies are boring.

Snacks for your pet Rust Monster?

what about give it something all the heavy armor are the same weight

Well, as far as D&D goes, 'iron' is baseline for performance considering statblocks for weapons and armor. For instance, using rules from various splatbooks, you can have weapons made from bronze, gold, or other materials, but then they would have penalties in relation to those made of iron. D&D is a game of abstraction (providing a semblance of rules for a heroic story) rather than simulation (attempting to model reality, given leniency for magic and monsters). For instance, 'iron' could be cast iron, wrought iron, various grades of steel (iron with a specific carbon content), not to mention bothering to consider whether the material was hardened, tempered, in what way, and how. All of those considerations would be taken into account during the smithing process for weapons and armor, and in reality would result in products which perform with some significant difference. Rather than bother with all of that, and to give leeway to DMs to include as much (or as little) of real-world considerations of metalwork, things not made of supernatural material (adamantine, mythril, cold iron, etc.) are given roughly the same statistics given their form. The point of having a Material (Iron) is to point out that it is not one of those aforementioned supernatural materials, but would be affected by D&D spells such as 'Heat Metal', which would not work on things labeled as Material (Wood), for example.