Title says it all. A Google search for "bank salt lake city po box" shows that many banks/CUs (Discover, Capital One, AMEX FSB, Schwab, etc...) accept deposits at a P.O. box in Salt Lake City.
Why are they all in Salt Lake City?
Many so-called "back office" functions have been transferred to Salt Lake City in recent years. Many people are surprised to learn that investment bank Goldman Sachs' second largest U.S. presence is now in Salt Lake City. So, much of the customer servicing personnel for certain industries are now based in SLC (even JetBlue, which is headquartered in New York, has many back-office functions performed in the Cottonwood Heights office in SLC), which could very well explain why you see addresses for so many financial institutions listed as SLC. It mainly has to do with cost cutting. If you were a company whose headquarters was in say, New York or California (with high labor costs), you might decide to open an office in a much more cost-efficient location (like SLC) and reduce your expenditures. Once one company starts to do something, many of their industry peers start to follow. Utah also has a number of state incentives to lure business to the state (not unlike Texas). Wages are also, per capita, much lower in SLC versus NYC, San Francisco, Chicago, et al.