Group is used to organize modeling, and it has no effect on the on-screen rendering of information.
Suppose you have a Form with dozens of fields, and you either want to simplify the model (the Form's model will look very crowded with dozens of elements), or you want to split the fields to logical groups (which can later be reused in other forms).
By encapsulating several field elements within a Group element, you can achieve a cleaner modeling with fewer elements under the Form, and it is also easier to reuse each of the field groups.
The rendering on screen is not affected - the display will be exactly the same as if the Group elements didn't exist (and the child elements of each Group were direct children of the Group's parent).
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