You want to summarize the prompt to demonstrate your understanding of it. Its a formality, as the interviewer knows the prompot, but its an important step
Restating the prompt might help you focus. Also, if you misunderstood the prompt, your interviewer will be able to correct you or redirect your response. Sometimes the point of the prompt can be ambiguous or misunderstood.
It seems to me that the prompt to describe yourself is so self-explanatory that it requires no introduction. I could see that you might repeat the prompt if it is a case that requires that you lay out the facts as you know them which would be what was provided in the prompt.
On the other hand, if you are confused and think that the prompt is x when it is y, I assume the interviewer would correct you.
In the MMIs I have had, after a quick introduction the interviewer will usually say something along the lines of "So, you have had a chance to read over the prompt. What is your opinion about X?" They kind of set up the conversation for you, so you're not just launching into your answer. The exception is role-playing scenarios, in which case you basically start as soon as you enter the room.
I say a few words to restate the prompt in each mini interview. This strategy helps me stay focused. It was also extremely beneficial in my last interview, because the prompts for two stations had been switched accidentally! After giving me a confused look, my interviewer was able to consult with the person running the interview to make sure the prompts were in the correct spots. So it can't hurt.
