I never really understood why any app has authentication on the iPhone. You have to authenticate (touchID) once to open your phone right? Other than if you leave it unlocked and someone picks it up (who leaves it unlocked?), why are we double unlocking?
Not everyone locks their phone.
It may work better for some to not lock their phone... especially if the only thing they want secure is their notes.
In my case... I use my iPad at work & often share with coworkers. Thus, I do NOT lock it.
Buuuuuut.... obviously, I don't want them to have (even inadvertent) access to my personal notes. This works well for me.