Going through the exercise alone, regularly - say twice per year - is extremely valuable, without needing to actually submit or save the answers in the application form they provide. The questions are nicely structured and hit on important points.
I hate ycombinator comments like this. Now I need to go procrastinate by writing a tool where I can go through the YC application every quarter.
Every quarter is a perhaps a bit excessive unless things are changing quickly. If you use a calendar, it takes a few minutes to just add a reminder at certain dates. Or I always just get reminded when I see YC posting about it.