ellipsis - Why is a preposition sometimes optional?

This question arises after seeing this thread.

Consider the following sentences -

1. 
   

   There is no point discussing this here (Between "point" and "discussing", there is a "in" and that is optional. And I am omitting it purposefully for asking my doubt)

   
   


2. 
   

   Do you see any point in filling up all the forms?

   
   
   


3. 
   

   There is no use filling up the forms. ("using "in" after "use" is optional here)

   
   


4. 
   

   What is the use of filling up these forms?

   
   

My question is -

Why in some sentences omitting prepositions like this is optional and why in some cases placing preposition is a must?