The old 2.2.5 explorer examples used FormPanel. So it was making an HTTP post? Preposterous. It used a FormPanel with a FormLayout, which conveniently allowed me in UiBinder to define a Field with a fieldLablel attribute that would be rendered into a FormLayout. The UiBinder markup is much more complex now, and this seems like a step backward.
The answer was fully correct and answered your question and even gave you input on when you should use FormPanel. There was no real reason why it used FormPanel. The GXT 2.2.5 example did not need to use FormPanel either. It could have used ContentPanel with a FormLayout also.
GXT 3 does not have any FormLayout anymore, that is why VerticalLayoutContainer is used. GXT 2 was not supporting UiBinder. However GXT 3 does and you can define a FieldLabel aswell as a width for each field using UiBinder.
You did not change it because you suddenly realized a HTTP POST wasn't needed, you changed it because FormLayout is no longer there. So actually your answer was wrong.
So, why in UiBinder is a field a child of a label? I guess the UiBinder complexity I referred to is a consequence of this strange design decision. Logically and semantically, this seems crazy. Why should a label have a widget as a child?
You did not change it because you suddenly realized a HTTP POST wasn't needed, you changed it because FormLayout is no longer there.
If that would be the reason, than we could still use FormPanel, because that is still there. You should only use FormPanel when you are really going to do a real form submit. Else you are adding unrequired markup to your application.
The example does not need FormPanel, so it is not used. However, you can archieve the same with using FormPanel. It is the same than in GXT 2. You could have archieved the same without using FormPanel.