I can't speak for the 787 only the other plastic jet the A350, the first thing I do on entering the flt deck is put the ADIRU's on, this allows, EFB comms with the ground, it allows IFE to be functioning and checked, it allows fuelling to be carried out plus a few other things as they all need ADIRU inputs, I can only guess that it may be somewhat similar on the 787, I could of course be totally wrong.