Custom CSS Box vs Child's Theme folder

I was able to get the Contact Widget Form working on my site.

However, my css changes were placed in the child’s folder in the member.css and then in the widget itself Custom CSS box and they both worked ok. Of course I deleted my changes from the member.css prior to placing it in the Custom CSS box.

But what’s the difference? Or a better question might be why would I want to place it in the widget itself as opposed to the child’s them folder?

Thanks