Containers pour a collection of widgets (and possibly child containers) into specific structures you like. If you want a form with labels on the left and fields on the right, you need a container. If you want OK and Cancel buttons to be beneath the rest of the form, next to one another, and flush to right side of the screen, you need a container. Just from a pure XML perspective, if you have multiple widgets (beyond RadioButton widgets in a RadioGroup), you need a container just to have a root element in which to place the widgets.
KeywordsSample Application Java Code Column Number Root Element Radio Button
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