Why did Rome hire mercenaries?

"to bolster the numbers of their army. plus they were considered an expendable asset. they would much rather have other people die than their own citizens"

This answer is mostly true; Rome would much rather foreigners die than their own citizens, but simplistic.

According to many Roman writers, including Tacitus and Julius Caesar himself, barbarian mercenaries were in many ways preferable to their own citizens in battle because the very civilization that Romans held so dear was conversely viewed as a means by which men were made "soft". The fiercest fighters, in their eyes, were barbarians, who had not yet learned to love the relative ease of city life, and were made rough by the savagery of their native lands. In addition, diets rich in protein, as opposed to carbohydrates, also meant barbarians, particularly northern barbarians, were generally much taller and larger in stature than their Roman counterparts.

It was also believed by many ancient Graeco-Roman scholars that cold weather made men fearsome and more courageous.

The Roman writer Vitruvius echoed the beliefs of many Greek writers before him, like Hippocrates and Posidonius, when he said, "... those races nearest to the southern half of the axis are of lower stature, with swarthy complexions, curly hair, black eyes, and little blood, on account of the sun. This poverty of blood makes them over-timid to stand up against the sWord... On the other hand, men born in cold countries are, indeed, ready to meet the shock of arms with great courage and without timidity".

It can also be said that barbarian mercenaries did not have the same biases and allegiances that many of the Roman soldiers had, meaning they were less likely to betray their leaders in support of a Roman rival. In this they were believed to be more loyal and dependable than Roman citizens, who had a greater interest in the intrigues of politics.