It comes down to sterilisation. Undesired yeasts/bacteria can develop undesired alternative fermentations and spoil the wine.Sulphur has been used for centuries for this. When I worked a summer in Germany in the sixties at a wine producer, they were still using the tradional way of sterilising wooden casks:set fire to a piece of sulphur-impregnated paper and drop it into the empty barrel. Sodium metabisulphite is just a more controlled way of doing it.

On the other hand, I've always been more suspicious of potassium sorbate used as a fermentation stopper rather than letting the fermentation run its natural course.