Loan officers don't approve loans!
Just as Realtors® don't approve acceptance of offers, loan officers don't approve loans. Lenders approve loans. In both cases, the Realtor® and the Loan Officer are acting as facilitators of the transaction. Neither of them has the authority to legally bind the parties they represent.
A Pre-Approval Letter is such a strong buyer's tool because it means that a Lender has reviewed the buyer's application and supporting documentation (pay stubs, bank statements, W2's etc.). In addition, the Lender has agreed in writing, that they will loan money to the buyer if they find an acceptable property. Loan officers can confirm to a buyer that they will be able to get a loan. The Lender will confirm that the buyer is actually approved for a loan. This approval will have loan conditions to satisfy, but it is a real approval. The predominant condition will of course be that the buyer finds a piece of acceptable real estate! Pre-approvals are issued without an address.
Your chance of being viewed as a Serious Buyer skyrockets when the seller and the Listing Agent are presented with tangible proof that you, as a buyer, prepared for this transaction in advance. The Pre-Approval letter indicates that you have taken the time to sit with a mortgage professional, discuss the details of your financial situation, applied with a Lender and been approved by that Lender, before looking at homes.
Understanding why Pre-approval Letters are crucial is necessary for real estate buyers. (opens in a new window)

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Mark,
Well put. It's always a good idea to keep the value of pre-approval versus pre-qualification before consumers and real estate professionals.
Fran