I have learned " 's " in English for possession and I know it is not common to say "the book of John" but when do you use "of" for possession? Is it for important things or people like the Tower of London and not London's Tower? Does it matter if I say the Financial Director or the Director of Finance?
The reason that it's the Tower of London, not London's Tower, is that it is a proper noun. Someone originally named it the Tower of London, so that is how it must remain. It is, of course, not the only tower in London, so it could be described as "one of London's towers" but in that case tower would begin with a lower case t.
As far as the job description is concerned, if you are talking about a specific person, then you would do well to find out what that person believes their job title to be. If you don't know, then the Director of Finance is just as good as the Financial Director.