What are the email fields in the order object used for?

The fields ctDynamicsSL.order.shipEmail and ctDynamicsSL.order.billEmail are used as placeHolders for future use or customizations.

When ctDynamicsSL.orders.getOrder(ordNbr) is called the system will attempt to load these variables using the following stored procedure calls:
xct_spDSLGetOrderEmailAddress @ordNbr, 'BILL'
xct_spDSLGetOrderEmailAddress @ordNbr, 'SHIP'

By default these fields will just be populated with a blank string unless a customization to the procedure is in place.

For information on how to save these values using custom sql calls see: How to make a custom SQL call with ctDynamicsSL.

Update 06/29/2011:

When ctDynamicsSL.orders.placeOrder() function is called.  The system will attempt to save them using the function ctDynamicsSL.orders.saveOrderEmailAddress.  This calls procedure: xct_spDSLSaveOrderEmailAddress which is meant for customizations as these fields do not directly correspond to an SL field.