Our earth sustains a large number of living organisms with large similarities as well as dissimilarities. To study each and every organism become tedious and cumbersome. So, to study them properly we classify them into groups and sub-groups.
In our daily life, we use many things like pen, pencil, copy, book; living organisms like a cat, dog, hen, cow, ox, etc.
If we look into them, we can sort out that a pen and a pencil are similar yet different. We use both for writing purposes but one makes permanent writing while the other make temporary marking, which can be erased. Here the differences and similarities help us in identification, procurement.