I'm not sure for Liberty exchange but for Mt.gox apparently you only needed that when buying large sums (+10k) using wire transfers.
Wire: If you plan to send more than 10000 USD as an International Wire, we need to know who you are. Please refer to the "Increasing Transaction Limits" section for more information on what to provide us with. Also, you will need to let us know 3 days prior to sending the funds so we can notify our bank and do not send. Please note that we are unable to accept funds coming from any entity or country listed by the OFAC
This snapshot is from 2012 but I doubt things were any different prior to that.