Your questions:
Usefulness of 4D polytopes ??
… by itself, not immediately clear. – But so are many other things, e.g.,
Earlier in 2017: News of “The largest known prime number:  2^74,207,281 - 1“ 
This number is  22,338,610 digits long.
It is estimated that there are about 10^17,425,163  prime numbers less then the next smaller known prime. 
That means we know only a fraction of 10^-14 of all the prime numbers in this neighborhood.
Is this knowledge useful?   Is mathematics useful?  
Often it takes several decades before a new insight in mathematics is used by some other discipline.

But even before this happens, mathematics can be beautiful.  By strict logical reasoning we can derive new mathematical insights that are irrefutable and not subject to later change.
By analogy from 1-, 2-, and 3-dimensional spaces we can derive 4 and higher-dimensional spaces.  And then we can show irrefutably that in 4-dimensional space there are exactly six totally regular polytopes.  No science or religion has this internal consistency and permanence.

All fields have some
Threshold Concepts
These are concepts that integrate prior understanding and transform it to a higher perspective.  They produce irreversible “AHA!” effects.  Examples from mathematics:
-	Notion of a function, say, y = func(x)
-	Limits:  sum of 1 + ˝ +1/4 + 1/8 + …
-	Euclid’s parallel lines  (no such things exist!  Even a light beam bends in the presence of gravity).
-	Concept of infinity
-	The infinite plane (Nothing we know is truly infinite).
-	The projective plane and understanding what happens when one goes through infinity.
-	Euclidean spaces of higher dimensions.
-	An unambiguous way of identifying and classifying symmetries.

Hopefully, by exposing you to some such threshold concepts , this will broaden your thinking and expand the style in which you may think.