Harrah's may use the info from it's slot card club for more than just marketing its services. Here's an example from Kansas:

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In the Topeka case, police brought a list of about a dozen names of city
employees to the State Gaming Agency and wanted to know when
those employees had visited Harrah's. The gaming agency had the
casino match the list of city employees with its "Total Gold" card
membership records to determine when the employees had visited the
casino, said Scott Stone, KAPE's legal counsel.

As a result, two employees were fired after "Total Gold" card information
was compared with each employee's record of leave time, he said.
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See the link below for the story in the Topeka, KS "The Capital Journal" newspaper.