Most mortgage lenders take the guess work out of applying for a loan by determining for you the amount you can afford to borrow. Then, they give you a printed document stating the maximum mortgage amount you qualify for based on your particular finances and income.
Mortgage pre-approval establishes your price range and strengthens your buying position by letting the seller know that you have already been approved for the loan. It can also ease time constraints once the purchase agreement is signed between buyer and seller.
You're out looking at homes. Your Real Estate Broker never mentions that you should get pre-approved and just ballparks what you can afford. You find the perfect house and work out a deal with the seller. Three weeks later, the lender informs you that the house is $10,000 over what you qualify for and does not approve your loan. The seller has already bought another house. You've given notice where you're renting and told all your friends about the great house you bought. And then, there's the money you've already spent on inspections on a house you can't own.
You and your Real Estate Agent have been working diligently finding that "perfect" home. A new listing comes on the market that's priced right and has got everything you've been looking for. You write an offer. Your Real Estate Agent takes it to the listing Real Estate Agent and is informed that another offer is coming in and will have to present both offers simultaneously to the Seller. The other Buyer is pre-approved for his loan. Whose offer do you think the seller will negotiate first??