Hi Colby, Unsubscribed Leads is a system smart list that is based on the qualification "Unsubscribed" is true. When Unsubscribed is true, Marketo will not email someone.
The benefit of adding the filter to the campaign is that they won't go into your calculated number as "blocked from mailing" as shown below. When there is a value other than 0 there, you will want to subtract that number from the "set to affect" number to get an accurate count of how many people this email will be sent to.