Ok that i ve realised is that the basic reason that the empire captured distant places such as morrowind is the fact that cyrodiil lacks of usefull mine materials.While we can see that there are lot of gold and silver mines(With whom the empire can maintain its armies), there is complete lack of materials such as ebony or glass.Now about the armor types some of them are named by the name of the material they are made(glass,ebony,iron etc).But armor types such as daedric or elven are not based on the metal they are made but on who first created them.There is a gap here on what kind of metal was used to create daedric, dwarven etc armor.I think it would be good if in a future game this was explained

Daedric armour are Ebony armour infused with the soul of a Daedra.
As for knowing what material the others were made of; I don't care.
Some kind of iron, probably.

And why would you conquer an entire contitent, just for some materials that have no real use, except decorating things (diamonds, rubies) or make weapons and armor (ebony, glass) ?

They have some important use because with them you make weapons.Also the more a material is rare, the more value it has.They expanded the empire in order to capture more exotic resources and to exploit them.I believe the expansion of the empire is basically for economical reasons.And this expansion also secures the empire from future problems.They control those mines in order to have a weapon control across the continent and to secure their leadership.

My english isnt my native language so mistakes are unavoidable

Elven is probably some sort of fine steel with a thin layer of gold on it. Dwemer armour is supposed to be made of 'brass', though this is most probably another substance than the brass we are used to.

Vvardenfell isn't a province, or a region... it's a pile of ebony and glass. Spot on.

Now just get your head out of the game-gutter and forget about 'high level armor sets.'