When a teat cup detaches mid-milking — whether the cow kicks it off or the operator removes it deliberately — an open connection to a live vacuum line becomes a problem for the whole system, not just that one cluster. Without isolation at that point, the sudden rush of air being pulled through the disconnected line drops vacuum pressure across the entire machine, disrupting milking at every other cluster that's still properly attached.
The shut-off valve is what prevents that: it closes off vacuum flow to a disconnected cluster automatically or via manual operation, keeping the rest of the system's vacuum stable while that one cluster is off. A worn or stuck valve that fails to seal properly defeats this protection — the telltale sign is a noticeable vacuum drop across the whole machine whenever a single cluster is removed, rather than the vacuum staying steady everywhere else as it should.