Originally, ransomware didn’t involve any data theft. Malware would encrypt the data on your computer, and demand a ransom for the encryption key. Now ransomware is increasingly involving both encryption and exfiltration. Brian Krebs wrote about this in December. It’s a further incentive for the victims to pay.
Recently, the aerospace company Visser Precision was hit by the DoppelPaymer ransomware. The company refused to pay, so the criminals leaked documents and data belonging to Visser Precision, Lockheed Martin, Boeing, SpaceX, the US Navy, and others.