When you're new, it's all about participating. Wow, I'm actually doing this! If you're really awesome, maybe it's about your ranking or a payday.
For the rest of us, competition is all about the data. Am I improving? Are my goals realistic? How do my results stack up overall? What changes do I need to make to my training plan?
It's hard to gage your performance in an open water swim. Conditions (wind, boats, currents, water temperatures, etc.) vary, and the course may not be measured exactly. If most people were 10% faster today than they were in the last race, a PR might not be very meaningful. But if most people were 10% slower, and you were only 1% slower, you've probably improved. (This is a nice post on the subject by marathon swimmer Evan Morrison.)
The data is what makes the entry fee worth it. The race swag and the Oreos are a lot of fun, but in the end it's all about getting the facts about how the entire field performed.
Next up: Race report from Saturday. (Still waiting for the official results. Times are off in the initial results...)
What's your reason for competing? How do you use the data from your races? Do you check to see how your peers do from one race to the next?