One of the most frustrating aspects of scanning film with most scanners 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 have the correct height to focus at and small differences can reduce the resolution of scans (if this is even possible).
Additionally, most flatbed scanners use rubber belts or plastic gears to move the scanning head across the film. This inevitably introduces geometric distortions which can be visible as squashed or wavy lines in the final scan.
When drum scanning, we mount the film to the surface of an optically perfect acrylic drum 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 metal worm screw which means that the scan is geometrically perfect. This is particularly important when people wish to composite images or stitch images together for panoramas.
Mounting the film for scanning like this is sometimes beneficial when trying to scan damaged film that is curled, kinked or bent.