Nanyang Museum is the main broad-history museum for a city with deep Han-era and regional associations. Its collections help place local sites such as the Medical Sage Temple, Marquis Wu memorial, and older administrative remains into a wider archaeological and cultural frame.
For most travellers it is a supporting stop rather than the reason to come to Nanyang. It becomes more valuable if you are using the city as a base for historical sights, because the museum gives names, timelines, and material culture to places that can otherwise feel disconnected.