1. Realtors prefer it.
2. Realtors LOVE when you have a letter of Pre-Approval, which will give you and your realtor leverage in negotiating with the seller.
3. Realtors prefer you go through the application process and document gathering, so you have a very accurate amount for the loan you can qualify for.
4. You do not have to lock into anything.
5. You do not have to buy a house that uses the full amount, the number will be the maximum you qualify for.
6. In some cases you can lock in an interest rate, if that is to your benefit in the current market.