Even though you are already approved for a loan, the listing agent for the home you are interested in may require you to go through the loan pre-approval process again, this time with their lender.
Listing agents often do this because they don't know your lender and want to make sure you truly can qualify for a loan.
The seller doesn't want to find out 3 weeks into escrow that you can't get a loan.
I personally do this 2 step approval process for all of my listings.
You would be surprised to see how many buyers don't pass the second approval process.