In 2012, there was an outbreak of food borne Hepatitis A in British Columbia. Investigators were able to track down the source of the outbreak but comparing the data collected on customers by grocery store loyalty card programs.
It turned out that the source of the outbreak was pomegranate seeds found in a frozen mixed fruit mix. The source of Hepatitis A outbreaks can be extremely hard to determine as the incubation period for the disease is 15 to 20 days, and who can remember what they were eating that long ago? The customer loyalty data gave investigators a solid lead in the early stage of the outbreak, when they only had six cases. Being able to track the source so quickly doubtless saved other people from becoming infected.
Certainly there are privacy concerns around stores collecting customer data like this. But here is one instance where it turned out to be to the public's benefit.