Hello,
"Why 12v booster, if all of your devices are no more than 5v ?"
Because the DRV8825 require >=9V as input voltage even if the coils for the steppers are only 3.6V.
So, 5V in, some components use 5V (or even 3.3V), 5V needs to be boosted up to 9+V for the stepper drivers which will step it down to 3.6V for the stepper motors!
Cyrille