What does high grade equipment do?

[lesserbob]

(Sorry if this is a double post. I tried before but couldnt see it)

So, Im going through equipment, and paticularly rare material armor.

I look at mithril armor. Less bulk, less sped penalty, lower str threshold. All makes sense.

But then, there is "standard" and "high" grade mithril armor. The difference? High grade armor is twenty times the cost, and thats all I can see.

So, why would I fork out the extra cost for mithril armor?
(Also, everything else. I see this all over the place)

[lesserbob]

Never mind. Found it on page 578.

It dictates hardness, HP, BT and max level of runes.

[Paul Watson]

Low Grade can only have +1 potency runes, and thus only 1 property rune.
Standard raises it to +2 and High Grade is +3.
The costs for using regular materials at high grade is assumed to not be that significant, IIRC.