Suppose something like a large grain of sand got between the tank and stand that I didn't know about. This could make a wobble point/point load. If there was something rigid, but squishy, between them, the material could squish around the obstruction.
Think of a fat person laying face down on a softball. Their back is still as flat as without the softball, the baseball has been absorbed into the "material".
I'm not trying to level an unlevel surface, I'm trying to provide for relief of small irregularities in what may be an uneven surface, either tiny imperfections in the surface itself, or anything that may get in there.
Even with a level surface, there may be places I could slide a piece of paper between the tank and stand, and the stand is therefore not properly supported. I want something to squish into those tiny voids.