User blog comment:VedranTheII/Pretentious Diaries 2: What tricks would you teach your Aura?/@comment-24994749-20150718160146/@comment-24993958-20150719100302

Jaune had his aura locked prior to Beacon, and Pyrrha was more curious than shocked, indicating that locked auras are the norm…if it didn’t have any drawbacks, Remnant would baptize kids by unlocking their aura since…you know, it would greatly increase their life span.

Grimm being able to better sense Hunters with their unlocked aura is the explanation that makes the most sense to me. So basically to be a hunter is to dedicate your life to fighting grimm by turning yourself into a bigger bait.