A Material's strength affects its utility as a building material. Materials like Foliage or Moss are unsuitable for building many structures because of their low material strength. Conversely, materials like Copper or Jasper are ideal building materials because of their great strength.
Tower strength is a measure of how high cube blocks of a material may be stacked before the tower collapses upon itself. Tower strength is basically a ratio of an item's weight versus its material strength. Moss is an example of an item with a very low tower strength as it begins to weaken at a height of 2 blocks, and becomes compeletly unstable at a height of 3 blocks.