Many people do not have HBO in the UK but in Europe and America it is one of the tv channels that has a varied range of movies but also tv series produced by them or by other companies. It has been a very successful tv channel over the years and has produced hit tv shows like Game of thrones, or The Sopranos.
Recently HBO announced their own online streaming service called HBO Go which offers exclusive content for viewers for 14 $ a month, it has unlimited access to all the movies and tv shows also, it is hd streaming and the subscription offers you the possibility to view from multiple devices.
It has a lot of content that only HBO streams and that can not be found on other streaming services and recently one of their tv shows Girls can only be found on the HBO channel or on HBO go, and its is not available for illegal download on any site, believe me i checked.
I think if they would give up on their the copyright restrictions and make the streaming services available for free the would not have as many subscribers and it would also be much easier for people to pirate the tv shows. Also considering the financial aspect they would not have the same income.