What's the functional difference between a form and a class based form?

I have seen code like the following enough times to wonder: Why not Do Form ???

release _oAnchorEditor
public _oAnchorEditor
_oAnchorEditor = newobject('AnchorEditor', 'AnchorEditor.vcx')
if vartype(_oAnchorEditor) = 'O'
	_oAnchorEditor.Show()
else
	release _oAnchorEditor
endif vartype(_oAnchorEditor) = 'O'

Thanks