You only need certification, be it as a welder, CPA, or nurse, if your customer requires it because they are looking for a level of comfort that you are competent to perform the work they need to have done.
Owners and architects require the fabricators to be certified to AISC, the Navy has their requirements that include an audit as does any certification program offered in the privet sector. Many state's bridge programs require their fabricators to hold AISC certification, but to a different level of competency than the building fabricator. There are industrial sectors that are not included in the AISC program and don't involve work for the military. They, just as building owners or bridge owners are looking for a system that provides a certain level of comfort that the welding shop has the capability to do the work required.
They are looking for suppliers that have a QC system in place, use qualified welders, know how to purchase materials to the proper specification, have inspection capabilities, etc. They don't fit the available certification programs such as AISC (the one I am familiar with). Who might fit this category? How about machinery fabricators or those companies that fabricate stairs? How about those companies that fabricate or manufacture sub-components for the medical industry or utilities that doesn't fall under ASME or AISC? Those are the types of companies the AWS Certified Welding Fabricator is intended to help. Again, it provides the purchaser a means of having an unbiased third party assess the capabilities of the fabricator or manufacturer to have a QC system implemented to help ensure the product is going to meet the requirements of the purchase documents.
Any company that doesn't have customers asking for some sort of certification clearly doesn't need this program if they are busy and are not looking to expand their customer base. However, if I were a fabricator, large or small, that was trying to expand the customer base, or needed a good marketing tool to show potential customers they have the capability to build a quality product, this is one avenue to consider.
A cash cow, no, I don't think so. The fees charged by AWS are minimal compared to some other programs that charge by the ton or by the gross receipts.
It's the same question as "Why does a welder need to be certified?" The answer is, "He doesn't unless it opens doors that would otherwise be closed because he wasn't certified."
Best regards - Al