one of the reasons why fook sao is important is because it teaches you proper hand / arm / elbow ending position / angles (not location) for tarn sao (lifting arms)
why is tarn sao important? Besides being a block and strike at the same time, tarn sao puts you in a position where you have you multiple options for what attack you want to do next.
as mentioned before practicing the fook sao places your hand in the proper ending position when performing tarn sao. So instead of having a straight arm when doing tarn sao by finishing tarn sao in the extended fook sao position, the ending position makes for easy transitioning (being that the elbow is at its proper 135 degree angle and the hand is bent at the wrist at a 45 degree angle ) from tarn to tan sao or tarn sao to completing huen sao to jut sao for example. By applying correct footwork (POCI) and being able to transition to either tan sao or heun sao, tarn sao give you options to go to the inside or the outside and that is the treasure
here’s an example. a right tarn sao can be done on the attackers right arm where you can transition to tan sao for an inside attack or that tarn sao can transition to a huen sao where you would put yourself in the position for an outside attack – correct footing (POCI) will need to be applied