Why create a child theme? (Once there is a customized theme)

Hi.

I′m using Snapshot Theme from WooThemes.

I didn′t know I should create a child theme, for customization.

Once I′ve customized the theme.. should I create a child theme?

I don′t know, which time there will be an update of the theme.

The child theme, has all the files of its parent theme.

Actually, what I just did, was.. a copy of the parent theme, for the child theme (and added the header text in ‘style.css’).

There′s a need to create a child theme, so if appears an update, I won′t lose the theme customization?

Is it correct, to use a child theme, with all files of its parent theme?

What should I do, so I won′t lose any theme customization?

Thanks.