The Issue Date is considered the creation date (by the default value that is applied). Users can change this value to suit there own needs, but it is an informational-only value.
 

The Send After Date is used to schedule email for future dates. This may be used because the author will be out-of-office for some period, but still needs a regular communication to occur, or an event marketing campaign is to be designed, where several email messages are designed in advance, and then each message sent on pre-determined dates. In the mail designer the user still clicks Send, but the mail server does not process the message until the date in the Send After Date field.