Question

Why do we need to classify elements give the answer in point wise

Answer 1

Classification of elements tells us about their chemical properties and nature
It tells about mass and size.
It tells about Ionization potential
It tells about electron enthalpy

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Answer 2

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..

Hope this helps.