It is possible for FINAL APP to engage before the FDP and if this does happen then it can lead to the aircraft not adhering to Altitude Constraints etc, hence the wanting in OM4:
“If APPR pb is pressed earlier, FINAL APP mode may engage. As a consequence,
resulting speed and altitude management in the FINAL APP may be inappropriate before
FDP”
As a technique it works really well to be level before arming FINAL APP inbound to the FDP, IT gives you the best chance avoiding undesirable behaviours. Arming in the descent inbound to the FDP is not prohibited but does come with risk, if you are aware of the risk and ready to intervene then that’s fine, but why take the risk.