My Cambridge application personal statement

I am applying for engineering at St Johns college and was wondering how important the personal statement bit of the "my cambridge" application is and what I should be writing in it.

If you've already covered everything in the personal statement, leave it blank. Don't repeat or put irrelevant stuff in their as you will be wasting their time. It's mainly for courses that are unique to Cambridge. My daughter left it completely blank last year and got in.