Why do you think geographers divide the earth into hemispheres?

There has been a tremendous historical significance to the hemispheres. The eastern hemisphere is the Old World from whence came the European colonists to the Western Hemisphere or New World (even the aboriginal people also came to the western hemisphere from the eastern hemisphere, they just did it some 30,000 years earlier than the Europeans). The vast Altantic and Pacific oceans make a natural geographic division of the world into east and west hemispheres, since north and south America are set apart by the oceans. The other hemispheric division of the world into north and south hemispheres also serves a purpose, since seasons are experienced differently depending upon whether you are in the north or the south hemisphere. Winter in the north hemisphere is summer in the south hemisphere, and summer in the north is winter in the south, so this is a meaningful distinction.