Used to run films in a past life.

The SDDS soundtrack (blue strips on the outer edge of the film) had severe issues with reliability the older the print got. This is because that part of the film is the bit that touches the rollers while the film is being transported from the platter to the projector and back so unless everything is squeaky clean and perfectly aligned it’ll get scratched and SDDS would begin to drop out (and fallback to other formats, usually analogue depending on your config) resulting in a bad experience.

If you got a second-hand print from somewhere you’d just never bother to run SDDS.