Because Albert Atterberg spent a lot of time making "worms" out of soil so you could live a safe and happy life. Albert Atterberg is one of the “Three Musketeers” of soil testing along with Arthur Casagrande and Karl Terzaghi. He developed this somewhat arcane test method that allows soil engineers to determine the moisture content limits where a silt or clay soil goes from one moisture state to another moisture state.
"What is this moisture state I speak of?" At low moisture the soil will behave as a solid, but with increasing moisture, it becomes plastic. With excess moisture it will flow like a liquid. This is very important when trying to build with or build on these type of materials. The two commonly determined Atterberg Limits represent the moisture contents at which a specific soil’s behavior changes from solid to plastic (Plastic Limit) and from plastic to liquid (Liquid Limit). The numerical difference between the liquid limit (LL) and the plastic limit (PL) is called the Plasticity Index or PI which is the moisture content range over which the soil will behave in a plastic manner.
In combination, the LL and the PI are used to differentiate (classify) silts and clays of high and low plasticity. In the field, the Atterberg Limits, can be used as a guide indicating how much a soil is likely to settle or consolidate under load. If the field moisture is near the liquid limit, a lot of settlement is likely. The opposite is true if the field moisture is near or below the plastic limit.
So when reviewing geotechnical testing data be “on guard” for Atterberg Limits.