Why do multi-cellular organisms need a transport system?

Multi-cellular organisms are too big and not all of their cells have contact with the external environment.  All organisms need to exchange substances with their environment - take in needed molecules and get rid of waste.  A uni-cellular organism can use the processes of osmosis, diffusion and active transport to transport substances in and out of the organism, which is in this case just one cell.  But as organisms increase in size, they need a specialized transport system to exchange substances between the internal environment and the external environment.  Interestingly enough, osmosis and diffusion are still the important mechanisms at the cellular level.