There are numerous organizations throughout the U.S. that have established their own rating systems. These are a great way to be able to make a quick comparison of others in your event (as opposed to the event not being rated at all). There are obvious reasons why many players choose to compete in USCF-rated events. Here is a list of those reasons.
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Here are three of the biggest non-USCF rating systems: Northwest Chess, Wisconsin Chess Federation, and CXR (Oklahoma, Missouri, and a number of other states).