The reason for child theme?

If you modify a theme directly and it is updated, then your modifications may be lost. By using a child theme you will ensure that your modifications are preserved. And you will be able to learn about the theme style.

Parent styles are for style changes that are common to all children. Child styles create variations on the parent. The decision to use a parent or child depends on the nature of the style change.

Only to make the update / upgrade process easier and not loose every times your theme customizations...
What else?

Thanx guys, it really bugged me for a long time. I'm trying to learn a bit. appreciated.