Shapur I's appointment as co-emperor alongside his father, Ardashir I, signified an important moment in the Sasanian Empire's governance structure. Ardashir I, the founder of the Sasanian dynasty, had successfully overthrown the last remnants of the Parthian Empire and sought to establish a stable rule over the newly formed empire. By bringing Shapur into a position of co-sovereignty, Ardashir strengthened the dynastic legitimacy and showcased the model of succession that would become a lasting feature of Sasanian governance.

As co-emperor, Shapur I was involved in diplomatic and military affairs, gaining invaluable experience that would later serve him well upon Ardashir's death. This collaboration marked a significant shift towards a more pronounced center of imperial power and helped facilitate the consolidation of land and resources across Persia. Shapur's early responsibilities illustrated the Sasanian commitment to a unified administrative strategy, which was crucial for managing the vast and diverse territories under their rule.

Following his father’s passing, Shapur I ascended to the throne as the sole emperor, continuing efforts to expand the empire, particularly through military campaigns against Rome. His initial co-rule was fundamental in shaping not only his reign but also the broader narratives of imperial expansion and governance that characterized the Sasanian era. The emphasis on dynastic sharing of power set a precedent for future Sasanian rulers, establishing a system that would influence succession and governance for generations to come.