wdDoModifyView method is a hook method which is called just before rendering the view.
This method can be used to access any UI element of the view for modifications. This access to view UI elements is only possible in this method since it takes a reference to the view itself as a parameter.
This method can also be used for dynamic UI generation.
Note that this method is called for the first time after wdDoInit and on every subsequent page refresh. This demarcation between first time calling of this method is handled through the parameter firstTime.
WDDOMODIFYVIEW is the method used to modify the view before rendering of the view. I would compare it do the PBO(Process before output) section of a classical dynpro. I would assume that you would be able to modify the UI dynamically in this method. You can also initialize the input fields.