There are three main types of seige weapons known as Catapults. They are Catapults, Balistas, and Trebuchets. All three of these were created only to help destroy and conquer, but they helped make way for our modern weapons. Two of the three weapons were designed after a smaller version of them. Those are the ballista and the trebuchet. The Catapult was an individual design.
The Ballista was designed by the Greeks after the cross bow. They were used to shoot over walls or at close up troops. They were used made after the tension string was created. Before that, there was no way for them to be easily made for fighting. They were too heavy to carry to battle, so they were made on site.
The Trebuchet was created in the 5th century by the Chinese. They used it in the sieges against neighbooring countries, and to protect villages. It was created after the sling. At that point, that was a powerfull weapon and just a single sling coupld throw a rock a good distance. So the Chinese tried to make a bigger version. It did not go as far as the others, but they still used it for protection. Sometimes a whole village would come together and use them to fight off invaders. This weapon was not used for long.
The Catapult was officially created by the Greeks, but were used during mideval times as well to take down castle walls. This weapon has a wider range of objects that are able to be thrown. Fire objects were used, arrows, and stone balls. This weapon was the one that stuck around the longest, even being used in World War I to send grenades across no mans land into enemy trenches.
In all, these weapons were the first seige weapons. They were the stepping stones to what weapons we have now. The people who made these weapons only needed them for the moment. But later they saw the potential they had, and decided to start making bigger and better seige weapons. Some died off, but some did not.