I just heard this story at a conference and it explains the purpose of assessment pretty well.

A boy tells his teacher, "I taught my dog how to speak yesterday." The teacher is surprised by this but is intrigued by the boy's confidence that he taught his dog how to speak. "Ok, why don't you bring him in tomorrow and show us?" The boy agrees and the next day brings his dog into the classroom. "Speak", says the boy. The dog continues to sit and sit despite the boy's desperate appeals to speak. The teacher says, "Well, I don't think he can speak." The boy replies, "I said I TAUGHT him how to speak, I never said he LEARNED it."
(Donna Gordon, 2011, BER Conference)

"Teaching is not learning."