Why do so many businesses register their permanent address as a PO Box in St. Louis?

Even when they operate in different states? I record people's employment in NJ, and over half of the business addresses I record are registered (with the federal id's) to MO.

Thanks to everyone who answered. I understand the benefits of using a PO Box, so I guess I'm asking why St. Louis?

I think it's just a coincidence that you've found several with PO boxes in St. Louis. Many corporations register in Delaware and Nevada for tax purposes, but they still usually have PO boxes in the city where the corporate headquarters is.

St. Louis is a really happening town?

alot of small business owners work out of their house and don't want to give that address, so that could be one reason.
Why different states do, could be they want their business to appear larger than it actually is or maybe MO has had the cheapest PO Box rental or their taxes could be cheaper too.

Well, they might be hiding something or the simple fact that they do not want spam newsletter...or they don't want their regular mailbox to overflow.