The down side is that you may pay application fees to cover the lender's paperwork in verifying your employment, income, assets, debts and credit rating.
You may select your own appraiser, or you may ask your real estate broker to help you with this task.
It means that you have in hand a lender's written commitment to put together a loan for you subject only to the particular house you want to buy passing the lender's appraisal.