Ask Hoshino Sendatsu the reason why we do Yamabushi training, and he’ll tell you we do Yamabushi training to do Yamabushi training. This answer is both insightful, and annoying.
In my opinion, Yamabushi training is to get you to focus on the now. A lot of the training we undertake is hurried, or there is an element of danger that means you have little chance to stop and smell the roses. This may sound counter-intuitive, but as a result we are forced to focus on our steps one by one, or each individual breath, and not to worry about those around us.
This reinforces the here, now, and self, of Zen Buddhism. This is essentially the same as the Haguro Shugendo philosophy of Uketamo, I accept, and is one important thing to take home with you after Yamabushi training. No point worrying about yesterday, it’s gone. No point worrying about tomorrow, not yet at least. All we have is the here, the now, and the self, and that’s what we should put our focus towards.