The ballista was one of the most effective and most valued weapons in the Roman arsenal. The ballista was generally used as an artillery weapon that would rain down large bolts on enemy positions so that troops would have an easier time attacking, similarly to how artillery was used in World War I.

Ballistae were used in many Roman battles, including the first invasion of Britain. The first invasion of Britain was fought between the Roman Republic under the rule of Julius Caesar allied with the Trinovantes and the Britons in 55 BC in an attempt to put an end to reinforcements sent from Britain to fight the Romans in Gaul. Caesar's forces landed around 11 kilometres from the Dubris (Dover), most likely at Walmer beach. The Romans were opposed by British chariots and cavalry, which made landing very difficult, what's more, the Roman ships were also too low in the water to land on the shore, and so troops were made to disembark in deep water while under fire from enemies in the shallows.  

To counter the fire from the Britons, Caesar ordered his warships that were fitted with catapults and ballistae to be run ashore from the enemies right flank. From this position, ballistae and other artillery could be used to drive the defending troops back.

The ballista was the perfect weapon for this scenario as their large bolts are accurate enough to be able to hit relatively small targets like chariots and cavalry from long distances, as opposed to a weapon like a catapult that can hurl object very far but with very low accuracy. The ballista played a crucial role in the first invasion of Britain for the Romans and would continue to play a big role in almost every Roman battle for centuries to come.

Links:

https://en.wikipedia.org/wiki/Caesar%27s_invasions_of_Britain#First_invasion_.2855_BC.29

https://en.wikipedia.org/wiki/Ballista

http://www.ancient.eu/article/649/