Why the aluminum cans in Earthships?

I'm just wondering what the significance of the slightly crushed aluminum cans are in the Earthships. Are they for any insulation value or just a filled for less concrete/cob? I'm working with cob and lime..wondering if I should be saving my cans or if it isn't really applicable to a full cob style home.

I don't know what earthship promoters say about aluminum cans, but as they are very conductive material even with airspaces in/between them, I doubt they would be much use as insulation. They would make an effective moisture wicking break in floors, or reduce mass considerably as fill, if used in those applications.

Since aluminum is highly recyclable, I would never put it in a building where it was not important to some function and better than available alternatives.

They are/were used as "bricks" to reduce the amount of concrete needed, as in the "forms" for the bond beams. And as infill between the tires.  

A recent Mike Reynolds interview he said they have switched to plastic bottles in most places--it was always about using trash that isn't being recycled not a particular material.