What is the necesssity of setting limits at the beginning of autocad?

The "limits" define the area of your "drawing board" (field). It is not neccessary to set them to form an inordinately large, or infinite field at the start of a drawing, as they can be altered later if needs be. It is neccessary to set them, however, as in certain circumstances the program uses zero as the default setting. If you blithely accept this setting you will have no field in which to produce your drawing.