It's one of those things that you arrive at yourself. You rarely, if ever, understand them from Haskell-ish mathematical-ish constructs like these.

Sometimes you could end up with this code in situations like this:

   const data = getDataForUser(user);
   const transformer = getTransformersForGDPR();
   const sanitizedData = transformer(data);

where getTransformersForGDPR returns a single function that may contain a chain of data transformations.

The example is contrived, of course, but you do end up with similar constructs especially if you have been exposed to functional programming.