Regulatory bodies require post-approval study data to help assure that there’s continued safety and efficacy and effectiveness of the approved drug or device or others. Sponsors’ failure to comply would incur heavy fines to market withdrawal.
Also, sometimes the drugs are approved on limited evidence which regulatory bodies would want to audit and review in toto, with “substantial” data to determine potential safety risk. Regulatories also insist and emphasize on having/generating post approval evidence for similar indications for which the drug was initially approved, which makes it a mandate for sponsors and in turn CROs to conduct such and confirm the benefits on the basis of the pre-approval trials.
Such trials are so designed to ensure well-established methodology and designed & conducted across efficiently, effectively.
They can be multi-site, global trials span over a period of years which includes controlled studies and data would be gathered to vindicate and justify the earlier approval or if not!