Anyone know the premise of this PCB assembly note?
It's clearly to get rid of moisture, probably to keep the steam from pushing BGAs or CSP packages off the board (maybe from moisture trapped in tented vias, microvias or other places), but I didn't look into all the possible reasons deeply. 

You might want to be a bit careful with this- 24 hours exceeds the baking guidelines in IPC-1601, so solderability may be adversely affected unless it's ENIG.