Child Theme

Hello, I was wondering about when to make a Child Theme. I am going to start to introduce some changes in the theme. My question is: - When is advisable to make a Child Theme and why? - Are child themes also updated with the theme updates? - If I introduce changes to the theme and the theme is updated do I risk loosing my changes? Waiting to hear from you and thanking you in advance, Kind regards, Frederico Pessanha