Sometimes they ask this because we may be iper-critical looking at them and push a noob to ask for other pieces without a real reason.
These are reps, so you should compare them with the gen BEFORE to buy, in order to see if their unavoidable flaws are acceptable or not for you.
Once you've chosen your model the QC pics are needed for mainly 2 reasons:
1) to be sure that the watch is going to be sent to you is the model you wanted (and believe me this is important!)
2) to check for real problems like scratches / bad assembly etc
If a newbie doesn't follow this path and post here a QC pic then we can start pointing out that "oh, the second hand is shorter then the gen" / "oh, the font of the date wheel is a bit bolder then the gen" etc, and he is moved to refuse the watch.
All these checks should be done by him in advance, especially with sites like Andrew's one that shows QC example pics, and therefore drive the decision to purchase or not a specific model in advance.
And even in this way it's not always easy: Rolex DayDate II exists in a multitude of dials combinations, also changed during the years. The TAG Gran Carrera cal 36 that I'm expecting from SuperMirrors has black seconds hand while at TAG official site it is red (but then you go on chrono24 to compare and discover that it exists gen also with black hand version)...