The show unique button when enabled your banner ad will only be shown to a visitor once per day. Meaning every banner impression is seen by a different visitor this feature has its benefits and of course its draw backs works really well for those who want to stretch there advertising dollars as far as they possibly can for branding purposes.
Also excellent for testing your banner ads and so many different other reasons.

Now the draw backs with your ads only being seen once per visitor your conversion rate may drop meaning actual clicks on your banner ad may decrease we want you to understand this feature works really well for some and not to much for others there's a lot of factors mainly its your banner itself that plays a huge part having the right content is key.