The use case is some sort of emergency that the driver may not have seen.

SF's Muni is in the process of onboarding new trams from Siemens. Someone recently stuck their hand in the doors as they were closing. On pretty much any newish tram or subway vehicle this will cause the doors to open again. It's shitty behavior, but that's what people do. Typically trams in the United States have door interlocks such that if the doors are open the vehicle doesn't move. The new Siemens cars apparently don't have well calibrated pinch sensors and the damn thing took off with some idiot woman stuck in the doors, dragging her down the platform.

I've had similar experiences with the older ones (where the drivers would deliberately disable the interlocks — this was before they put tamper evident bits on the override switch). The driver closed the doors as I was boarding. Without looking at his/her mirrors the driver took off as I had one foot on the street and one on the vehicle.

Same deal with people who've fallen onto the tracks.