Acura, I'm not really sure who I would go ask who could tell me exactly why they want the applicants to submit this 300 word admission essay. Did you guys have to still do the supplamental application that had a few short response requestions?
I will tell you what I know though that may help you out. As most of you are aware the process of getting selected into pharmacy schools is not only based on GPA. On thing USC makes a big deal about is you have to be an effective communicator- this means oral and written. One of our professors made a big deal in class the other day how great the writing has improved at our school over the last couple decades. Anyway, so don't worry too much about what they are going to do with this little pre-interview essay you turn in. Just make sure you believe whatever it is you are writing so that if somehow it came up in an interview you can talk about it smoothly. However, they probably haven't changed the interview format, so most likely it'll still be closed file, which basically means, they are probably going to take this essay and read it, give you a score for it, put it in their computers, and move on.
One big tip I will give anyone going in for an interview is know some of the issues going on in pharmacy, or the health care system in general (medicare part D etc). A big question could be "what do you think is one of the problems facing pharmacy right now?"
Good luck guys.