Why does my Mortgage Broker need a full disclosure for a pre-approval?

Why does my Mortgage Broker need a full disclosure for a pre-approval? - 1
Added (1). I just want to know if this is typical because I am not very familiar with the terms and conditions for a pre-approval and I just don't want to end up signing a document that I don't what it is for. Could someone please explain to me why this is needed? The mortgage broker said it is because I have only worked at my employer for less than a year and that the underwriter is wanting this, due to the fact that I get paid overtime and commission.
Added (2). Also, I want to add, that I already submitted all my W-2's, payroll, 401K & IRA statements, bank statements, and identification copies. After these were received I was asked for a borrowers authorization form to be signed, to get employment/income verification.

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