On the way to the crucifixion, the soldiers compel Simon of Cyrene to carry Jesus' cross. Simon is not introduced as a volunteer. His sudden involvement reflects the coercive authority available to Roman forces.
The episode gives a concrete example of compulsory service—the same social reality behind Jesus' earlier image of being compelled to go a mile.
Military and governmental personnel in the Roman world could requisition labor, animals, lodging, or transport under various circumstances. The Gospel verb used for compelling Simon is associated with such imposed service.
Condemned prisoners could be made to carry the crossbeam or other part of the execution apparatus to the place of crucifixion. The Gospel accounts do not require the familiar artistic picture of Simon carrying an entire assembled cross.
The context explains why a passerby can abruptly become part of a Roman execution procession. Simon's preferences are irrelevant to the soldiers who press him into service.
The scene also gives the language of cross-bearing a stark physical reality: before the cross became a revered Christian symbol, it belonged to Rome's machinery of shame, coercion, and execution.