I'm afraid that there is not a great deal to say- we invert subject and verb after certain words and phrases- some adverbs (never, seldom) and some negative phrases (not only, not once). Some say that changing the word order allows for a change in emphasis. It is also fairly arbitrary- we can say 'never have I', but we don't use 'always have I'.
However, they do benefit teachers as they keep us in a job teaching them to advanced learners. There's also the groupd where the second verb inverts:
Only when you are Italian, are you perfect.
Student or Learner