The reason for dividing the forces and marching south is that Spartacus and the other leaders considered how massive the rebel army had grown. If they stayed clustered together, it would not only hinder their movement but also make gathering sufficient supplies along the way difficult, potentially leading to internal conflicts. Additionally, splitting the forces and advancing south would further expand the rebel army's power.
Of course, there's another important reason the rebel army chose Benevento as the place to divide their forces — two major roads leading south converge here.
One is the Appian Way, which runs from Benevento through Tarentum all the way to Brindisi. This is one of the earliest roads Rome built to connect to Great Greece. Back then, King Pyrrhus of Epirus led his mercenary army through this road to march on Rome. Spartacus and the leaders' army planned to take this road.
