To remind you that you are INDEED stupid and that your prep course has a built in 'higher score guarantee.'
I put 0 stock into diagnostic. I suppose they let you know what your real weaknesses are, but for me at least:
-I correctly selected my weaknesses before hand (test confirmed)
-It let me know that I had to review everything (this is why I was taking a course)
-I also feel it was absurdly difficult compared to other exams - higher score guarantee
-I think it freaks a lot more people out than it really helps
Reviewing practice full lengths is absolutely golden, but a FL before any content review is just pointless in my opinion.