Of all the years where I have read institutional letters, it usually falls on the committee letter writer to present the personal background, not the applicant. I don't know if this is the way this committee wants to save time writing the packet or they feel more comfortable taking a perspective on just the academic performance of the candidate. Yes, when I have written committee letters, I have asked for a biography as well, and sometimes there are statements I would quote that I felt I could not capture adequately. But I gave everyone one page at the most. Three pages single-spaced in smaller than 12 point font are turnoffs (wall of text).
I'll differ with
@Goro here until more information is provided, but like the essays for institutional actions/criminal history, you need to get that autobiography vetted very carefully. It also provides a trap to rehash information that you could discuss in a more focused fashion elsewhere in the application (such as the disadvantaged statement). What is the prompt for your autobiographical statement, and what suggestions are made on what you may include?