Go ahead, just try detailing four sets of grandparents, your parents, yourself and spouse(s), and your children as both parent and child, over and over again. On paper its hard.  You'll soon get muddled and the pages out of sequence.  Especially when researching and writing at the same time. But on a computer its easy.  

Genealogy software allows you to file information in the right place the moment you find it.  That saves hours of re-formatting or re-writing on paper later.  Genealogy software makes it easy to add new information whenever you find it, correct your mistakes, backup your data for security or sharing, track progress throughout your hunt, and keep it all neatly filed in a convenient place.

Most modern genealogy software will print family trees and charts, generate automatically-indexed books on demand, turn your findings into coherent web pages, and make documenting your sources a snap.

Most genealogy programs cost less than $100.  Some are free. The only justification for not using a genealogy program to record your genealogy is that you would rather work harder.