Hi
This is because earlier old 320 aircrafts have to do this ,they did int have GPS primary & u knw its for T/O with intersection rwys & u also have T/O inhibit which takes care of ur all small T/O failures ,But it is good to do it because If u move on the Wide body 330 then it's mandatory to do it as it will give an ECAM on the T/O
Cheers
