They do this because repair fraud is still a huge problem in China. If they take your laptop or iPhone into a back room to repair it, 9 times out of 10 they are going to swap out other components with faulty ones (hence getting more repair business down the line along with a non faulty component to use later). I’ve been caught by that scam once in Beijing when I needed my baseband replaced, they swapped out the entire internals and it broke again a week later.

Everyone in the know insists on being there for the repairs to make sure no shenanigans occur. So the reputable places do it in front of you and explain what they are doing so you don’t feel cheated later if something else breaks.