In order to study elements in an organized manner .The need to classiffy elements into families with similar properties arose because, lacking this basic tool, the chemist had to deal with each of the 45 or so known elements one at a time, as if there were no rhyme or reason to them...which made things very difficult.But there were similarities, and so elements began to be grouped into families with similar properties.
This allowed chemists to predict the properties of elements which had not been discovered...about half the periodic table, based upon our current knowledge of the natural elements..
