They were the only plays performed at that time. Materials were from the communist activities or related subjects. Actually the then staged modern plays were not just limited to eight. Usually that means adopting the playscripts as they were and changing the music accordingly. Modern plays can be understood as a trial of reform to this grand old art.
Traditional plays feature the stories of ancient or pre-modern Chinese history.
There are certain forms for the costumes, the character face paintings, stage properties, musical melodies and stage actions. But most of these are not appliable to the modern plays. For example, modern weapons, such as pistols and rifles, are used on the stage in stead of pears and swords. Therefore new performaning forms were developed. The most famous change in the modern plays is the use of a huge orchestra including violins, piano etc instead of the usually small musical group, producing the most beautiful sounding effect.