First, a pre-approval letter is not the same as a pre-qualification letter. A pre-qualification letter can be generated in a few minutes by calling a lender, or on line, and is based on information you provide and a quick credit check. A pre-approval letter, on the other hand, requires verification of income, employment and financial information. It is an actual approval of you, the buyer, subject to the property meeting lender appraisal guidelines.
A lender is not bound by the pre-approval letter, but as long as your financial condition does not change in an adverse way, or the time commitment has not lapsed, it is deemed dependable. Note that interest rates could change too, which may affect the terms of the pre-approval . . . good or bad.
Why is a pre-approval letter a must?