They use this to search profiles as they're registered, to eliminate repeat users who have abused the site in the past. (adultdating.net)
BUT «Repeat users of legal services are not commonplace, so...» — spoken like a barrister! (slaw.ca)
When I was in private (solicitor's) practice, for eight years, I had lots of clients for long stretches or the whole time (clients of the firm, in many cases, but repeat users regardless.) (slaw.ca)