There are a variety of chopping techniques in cooking.
The goal is to have the vegetables in a size appropriate to the cooking time that the recipe being prepared allows for their cooking time. It also allows the vegetables, if chopped small enough, to be more easily integrated into sauces or to sit lightly as a garnish on top of something such as a soup.
There is another, unstated goal behind the techniques. The uniformity of the resultant chopped vegetable or fruit is seen as a mark of how professional the cook that did the chopping is.
The precise names allow a recipe writer to dictate to the reader, or a chef to dictate to his or her underlings, what size of chopped vegetable s/he wants without having to state the measurements or say how to do it.
That being said, note that the meaning of some terms may vary from 1 to 3 mm (1/16th inch to ⅛th inch), etc., depending upon the interpretation that a particular chef or teacher has put upon the variations allowed for a cut.