He was delivering a message with the watch. He had to be there so the person understood the meaning of the watch. ALso the writers needed a reason to put him on the plane!
Jug a much better explanation then mine
He had the watch on his lapnot i hs luggage, he coudln't amil it, Jin ahd ot deliver it with the threat don't try and take offf we are watching yo,paraphrasing.So knowing what Sun's dad does, wht wa sin that watch? A timer device to blow up?