Because Quisition tests you on cards at the optimal interval for retention, it's important to come back to the site when cards are due. While Quisition has always told you how long before this is the case, it's easy to forget to come back to the site at the right time.
With e-mail reminders, you'll get a regular e-mail when its time to review cards. This reminder can be turned off at any time from the Reminder Settings section of the My Decks page. Note that you have to have a verified e-mail address in order to receive e-mail reminders.