What did he do for he Dutch history?
I think he didn't do thing aspecially for the Dutch history but that he helped some people with his music and that's the reoson he became as populair as he was. In this years you still see poeple who listen to his music, who buys his paintings, some REAL fans! There are lots of pages about him, a biography and lots of information. Herman was a true hero for some Dutch human beings! 
If you're visiting The Netherlands you have to know that his nickname was Rock 'n Roll junkie and that he did a lot of drugs and alcohol. You have to know that that the reoson was he commited suicide. And the main thing that you have to know is what kind of music he made.