This Condition located on the Ministry category tab in Search Builder allows you to search for everyone who received a specific email. You will enter the Email ID # as the Value. You can find the email ID in the URL when you are on the Email Details page. Enter that number as the value in the Condition.
Use Case 1 - Follow-Up Email
A good reason for using this Condition would be to send a follow-up email to the same group of recipients who received an initial email. If you sent your original email to an organization or search results there might be some people no longer in either of those. However, if you use the email ID # to find the original recipients and then send your follow-up email to that group of people, you will be ensured that the exact group of recipients will receive your second email.
Use Case 2 - Evaluating Responses
Another reason for finding email recipients would be to see if they responded to the message in your original email. For example, you send an email inviting people to an event. By finding the group of recipients and adding the Condition Is Member Of or Attended As Of and selecting the specific organization and / or date you would find just those people who received your email and attended the event or joined the organization.