On the one hand it means nothing. On the other hand it's pretty unusual for a person to not be able to get such a document from a mortgage company. As such, not having one means more than having one does.
It seems to be an important document to the seller's agent indicating you're not so credit unworthy that you couldn't even get a pre-approval letter. It is also important to them because it tells them who you plan on working with for the mortgage. They may or may not like your choice. The sellers' agents I worked with uniformly did not like my choice of USAA. They wanted a local bank originating the loan.
The letter helps with having your offer looked at at all. No letter probably means you get ignored.