It depends on the circumstances
If you are visiting with a lady in the US, the photo ID is to ensure you are in fact the lady she screened.
In Canada, there are a few reasons why a lady would want that information. If you mentioned that you were young (19 or so) or if you appear to be quite young, she will want to ensure that you are in fact of legal age.
If you used a Credit card to pay for part or all of the tryst, she may want the photo ID as additional security to verify that you in fact did authorize payment.
Those are the only reasons I can personally think of - though I'm sure there are many more. Each lady works within her own paramater of comfort, and perhaps the lady whom you are choosing to visit is more stringent than many in Canada. If you really want to know, you can always ask her.