The user wants to leave an opinion or evaluation about an object, person, place or thing. Quick opinions can be captured using ratings or voting ("thumb's up," "I like this!"), and more in-depth evaluations can be captured as reviews.
A user wants to quickly leave their opinion on an object, with minimal interruption to any other task flow they are involved in.
User wants to share her opinion with others about an object (place, person, thing) in greater detail than a simple rating.