I think the new KA flight is kind of useless, as why CX just don't upgrade one of their MNL flights to a B773 instead.

But a reasoning for adding this flight is that there is a huge gap between the 4:30pm and 10pm flight to Manila, and the KA flight fits perfectly into that gap which offers more options. Similary, on the return, there is a gap between 6:30am at 11:05am and KA's flight sits in between. Also notice that the KA flight does run on day 2/6 on HKG-MNL and 3/7 on MNL-HKG. That is because CX operates a extra flight on those days that KA doesn't operate.