I had the experience of having both.
The first slice means no one has had any before you. It shows the promise of a magical experience that no one else has had yet.
The last piece means everyone before you got some, but thankfully there is one piece left just for you.
In both cases you have no idea how long the product has been on display. Is the whole cake straight from the fridge and ice cold? Has the last piece been on the plate, passed over by the masses, drying and waiting for a wishful sale?
There’s a perceived probability in both.
Both are buying opportunities.
Both highlight the hazards in perception.