The lighter pieces like "Scorpions", small ballistas, were deployed on battlefields if the situation and terrain permitted. Their role was more to demoralize and annoy an enemy (particularly "barbarian" units) from long range rather than to inflict serious casualties. The were small enough to be relatively portable and quick to set up.
Source: Adrian Goldsworthy, The Complete Roman Army
It was indeed!
I am not a military historian, so perhaps someone with more knowledge can elaborate, but pre-modern artillery was certainly employed in field battles, though much more rarely than it was used in siege operations.
Alexander the Great famously used catapults to protect a river crossing at the Battle of Jaxartes because they had a longer range than bowmen. This is (supposedly) the first instance of field artillery being used this way, but I can neither confirm nor deny the veracity of that.
Caesar also used siege engines mounted on his warships to help secure a beachhead during his invasion of Britain in 55 BCE:
When Caesar observed this, he ordered the ships of war, the appearance of which was somewhat strange to the barbarians and the motion more ready for service, to be withdrawn a little from the transport vessels, and to be propelled by their oars, and be stationed toward the open flank of the enemy, and the enemy to be beaten off and driven away, with slings, arrows, and engines: which plan was of great service to our men; for the barbarians being startled by the form of our ships and the motions of our oars and the nature of our engines, which was strange to them, stopped, and shortly after retreated a little.
-- Caesar, Commentarii de Bello Gallico 4.25
The Roman and Byzantine armies also used smaller versions of ballistae in various forms, including the scorpio and chariot-mounted carroballistae, in field operations. Here's a particularly graphic description from Procopius, if you're in to that sort of thing:
And at the Salarian Gate a Goth of goodly stature and a capable warrior, wearing a corselet and having a helmet on his head, a man who was of no mean station in the Gothic nation, refused to remain in the ranks with his comrades, but stood by a tree and kept shooting many missiles at the parapet. But this man by some chance was hit by a missile from an engine which was on a tower at his left. And passing through the corselet and the body of the man, the missile sank more than half its length into the tree, and pinning him to the spot where it entered the tree, it suspended him there a corpse. And when this was seen by the Goths they fell into great fear, and getting outside the range of missiles, they still remained in line, but no longer harassed those on the wall.
-- Procopius, The Gothic War, XXIII
I hope that answers your question! In general, artillery were far more useful in siege operations than field engagements, but pre-modern military commanders could be a pretty inventive bunch.
The earliest use of artillery in pitched battle dates to 353 BC. The Phocian Onomarchus drew his Macedonian enemies into a narrow place ringed by a horseshoe of hills, on which he had placed stone-throwing artillery. Once the enemy army came into range, their lines were disrupted by a shower of stones, and they fled. This unnamed battle was the only major setback in the spectacular military career of Philip II of Macedon.
Unfortunately, the only source for this battle (other than an unspecified mention in Diodorus of Sicily, Library of History 16.35.2) is the very late and questionable testimony of Polyaenus (Stratagems 2.38.2).