The virus "Petya" attacked more than two dozen countries

 An excerpt from the article: 

•  Most of all from the virus, which gave the name "Petya", Ukraine suffered, everything started from it. There were blocked sites of banks, energy, transport companies. The extortion program requires a ransom - $ 300 in bitokoyah. Judging by the electronic wallet, which is indicated for the transfer of funds, paid nine users around the world. 

 Link to entire article: https://vk.com/all_news_worlds?w=wall-62287548_9044 

 Why did hackers make this virus?  What purpose do they pursue?