Why should you get pre-approved before looking at homes? The biggest reason, is that you will be conveying to sellers, that you are a legitimate buyer who is serious about purchasing a home since you have already taken steps to start your process with your lender. Also,you’ll know that the homes you are previewing are ones that you can realistically afford. If you start looking at homes that are in the $350,000 range first, than after getting pre-approved for around $125,000. You will severely change what you are looking at. Also, by doing your homework up front, you will potentially save headaches down the road through the buying process.
Check out this article from bank rate for more information about the pre-approval process.