I was wondering what the bottom text box was for.

The usual UX for check boxes is to toggle when the text of the control is clicked.

Because of that I avoid clicking on check boxes that I do not know if I want to change. Hence I did not get to see the description of the options.

The PKG installer is odd in not having that behaviour.

To check I remember this correctly I use Apple Mail Preferences
and click the text on a check box item.