The Prophet (may Allah's peace and blessings be upon him) clarifies for us the evil types of earnings, so that we would avoid them and resort to lawful ways of earnings instead. The money earned from selling dogs, the money earned by a prostitute and the money earned by the cupper are examples of such evil earnings. Note that the word translated as evil here is 'Khabīth' and has many meanings including evil, malicious, disgusting, offensive, etc.