I'll just guess that their email marketing database is separate from their users database and there is an update process that runs nightly to add or remove cont

I'll just guess that their email marketing database is separate from their users database and there is an update process that runs nightly to add or remove contacts and their preferences. However if there is a problem during the update, they want to be sure they have given a large enough window of time to correct it without either exceeding the time frame given to the user or having to shut down emailing until the problem is fixed. The other possibility is that they queue up email jobs for approval a couple days in advance instead of pulling the data from a live system when it is time to go out.