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Just a guess, but it might be to help control humidity. In some of my readings on meat smoking/curing, some people use a big pile of salt to control humidity in a refrigerator. They take a large pan with some water in the bottom and add a big pile of salt in the middle... as the humidity in the room changes, the salt pile will either absorb more water or more water will evaporate to stabilize the humidity... or something like that
If you google "saturated salt solution humidity" you can read more.
The first thing that came to mind was Himalayan salt blocks. So I bet it's used for thermal regulation. Like a water pan, only different. But then, it doesn't seem like enough to make much of a difference.
Another thing to watch for when building a smoker... not all plywoods are suitable to use for cooking foods!! Be careful and check your materials before building.