No, it is not absolutely necessary, but I don't recommend it either. The only time an offer should be submitted without a pre-approval letter is when you are making an ALL CASH offer.
A pre-approval letter shows the seller that you are a serious buyer and that you have the wherewithal to actually buy their property. Without the letter, they can't really consider your offer.
Additionally, there may be other offers that the seller is reviewing. Chances are that the other offers include pre-approval letters, and therefore, the sellers will likely move forward with them first. In my opinion, every good
