Why ATTI mode?
1. When IR sensors or GPS lock not functional.   Right now it seems the Neo goes into "fly away" mode if lighting is insufficent.
2. Indoors flying for the reasons noted in #1.  
3. It is basically non-dji angle mode.  A mode  that most can easily master and many have.
4. When GPS sensor for whatever reasons fail or experience significant interference.  If you have developed the skill to fly angle mode you can continue to control the quad.
5. Transitional step to full manual flying for those interested.

FPV RC3 set to manual and keeping the Atitude feature engaged seems to be the only option.  

From my just brief google search.  DJI use to have an option through a switch on the Tx/controller to engage ATTI mode.   Future option to add ATTI mode function to one of the custom switchs on the various controllers.  Neo would become an improved platform as a manual training quad.

100% agree, what we call "angle" mode on custom FPV drones is an absolute must for Neo considering it's marketed as an indoor-friendly drone but its vision positioning is so bad that it doesn't really work.