You don't need a stylus. It is just a personal preference, and in most cases only for specific apps.
We've all spent a fair amount of our life building motor skills with pens an pencils. That means that for some tasks, like drawing and writing, most people are more comfortable and skilled with a stylus.
A stylus may also offer some increase in precision, also handy for drawing and writing. They are not anywhere near as precise as old resistive screen styluses, but you take what you can get.