The Meaning of Life, the Universe, and Everything.
Hmmm. Other forum guys, correct me if I am wrong here, but I believe that is the texture that Minecraft will default to if you don't include a texture for a certain block. For example, if you are missing the lilypad texture, the lilypad texture will default to the missing_tile structure.
This is Bedrock's equivalent of the "missing texture" icon, which is used for the placeholder block used for invalid block IDs (Java simply deletes such blocks so missing texture blocks only occur when the texture or block model is missing or invalid. The Wiki indicates that Bedrock also has the pink/black checkered missing texture for valid blocks with missing textures):