DIY Slate wall

Sorry if it's been asked (it's a long thread), what's the purpose of the styrofoam on the bottom?

4 1/2 year old thread... not sure if any of them are still around to answer that LOL

My guess is so as just not to have to use so much slate and to make it lighter and easier to move if/when it needs to be.
 
I was hoping they'd see it and still be around. I found this in the DIY sticky.
 
But what's the point of the styrofoam on the bottom? I have some pink/rose slate that I'd like to make a wall or something out of.
 
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