As usual, it depends
Personally, after a calibration/setup, I leave the subs themselves alone. If, however, you would like to use the three PEQ (or distance change, whatnot) along with REW for experimenting, then the app is
very useful, especially if you have multiple subwoofers. Even setting the volume during calibration the app is handy as the back of the subwoofer might not be that accessible.
For most users the app would not matter that much, but for the tinkers, I think its a a gift from God, where it not for that fact I'm an atheist