A pre-approval involves actually verifying your credit, down payment, employment history, etc. Your loan application is submitted to an underwriter and a decision is made regarding your loan application. Getting your loan pre-approval allows you to close very quickly when you do find a house. A pre-approval can help you negotiate a better price with the seller, since being pre-approved is very close to having cash in the bank to pay for the house!
If you can get a pre-approval letter, your offer will be viewed far more favorably.