It is not mandatory from most agents, however it will definitely show the agent that you are indeed a serious buyer. Also, having a pre-approval letter issued from a lender GREATLY improves your ability to negotiate a better deal with the seller, and really streamlines the time it takes to reach a signed deal! If you find the perfect house, you really don’t want to wait a day or two just to get a pre-approval letter (the house may be sold to someone else in the meantime!) With all that being said, why WOULDN’T you want to have a pre-approval letter, right???