When you are pre-approved, it IS based on accurate information so it is a real number that you can get approved for, as long as your circumstances do not change.
Realtors know that being pre-approved means you will be able to get a loan for the amount you are pre-approved for.
So realtors have confidence that they are showing you homes in the right price range.
They also can negotiate with the sellers based on your pre-approval, which can often make the difference between your offer getting accepted over others.
Can speed up the buying process.