Scanning film produces superior results that can be achieved from scanning a print of the same image. There are several advantages to scanning negatives and slides:
Film (a negative or slide) is the original image, whereas a print is a second-generation copy.
Film provides a greater dynamic range (contrast) than is provided by prints.
When the photograph is printed, some tonal range and color information are lost or modified. The original information can't be recovered from a print. However, the original film will still contain the information and yield more detail in a film scan.
This doesn't mean that you shouldn't scan color photograph prints—just that scanning film produces noticeably better output than is possible ...