One of the most frustrating aspects of scanning film is keeping the film flat. Even slight curvature in the film that is almost invisible can cause the edges of the scan to be soft. Many scanners also have to be set up to the correct height and small differences can reduce the resolution of scans (on some scanners, height adjustment is not even possible).
Additionally, most flatbed scanners use rubber belts or plastic gears to move the scanning head across the film. The slack in this system inevitably introduces geometric distortions which can be visible as squashed or wavy lines in the final scan or in false colour fringing.
When drum scanning, we mount the film to the surface of an optically perfect acrylic drum (which each retail in the thousands of pounds) so that the film is held completely flat in relation to the scanning head. The scanning head moves across the film through the use of a zero-backlash metal worm screw which means that the scan is geometrically perfect as well. This is particularly important when people wish to composite images or stitch images together for greater resolution.
Mounting the film for scanning like this is sometimes beneficial when trying to scan damaged film that is curled, kinked or bent.