Why should you fill in a YC application even if you know you wont be accepted?

Because you'll discover lots of stuff you thought you new but didn't know. Your unknown, unknown. Your odds wont go up by much but your enthusiasm to get there will give you that laser focus, which you need, and more importantly the fact that it can never be good enough. I've edited my biotech application maybe like probably > 70 times, some were big dribly bit, some only small ones, but my vision is now not just that more condensed, its becoming crystal clear.