I can understand the need for adhesive for wall tiles. But if you have a flat floor, and medium to large tiles, why can't you just lay your floor tiles on the floor without any thinset and grout it. Each tile is fairly heavy so surely that will ensure it stays in place. Also the grout will reduce any movement?

I can't seem to find a reasonable answer as to why it is always necessary to put thinset if the floor is completely flat and tiles are medium to large making them quite heavy.