Choy Lit Fut person throwing a "Pow"
Only people that understand CLF as you (PC edit: sometimes appear to) understand wing chun would seek to do this. That motion is done for a specific reason, probably not what you think.
Anyway:
As he steps forward to punch (because he is already in range)...perhaps with some angling on his footwork....he sees that wide punch coming in his peripheral vision so his rear hand automatically comes up to cover it AS he is punching. If his punch connects before the opponent's strike meets his Tan, it may never even make contact because he has stopped the person in progress....or it may continue swinging because it already has momentum and the Tan Sau keeps it from making even trivial contact....or the opponent may step back and angle just enough that your punch is not as effectively as you would have hoped and the opponent's strike is still coming with force. But because your Tan is up you are covered. Or....he may back off just enough and covert his wide high punch to a lower punch in route and you simply switch your Tan to a Gan and continue moving in with your punch.
This is (PC edit: a scenario somewhat more likely to be played out in fiction than reality). The chance of you putting out a tan in time to react to an incoming hook is (PC edit: very small). The chance of you converting that tan to anything else en route is (PC edit: very very small). You are (PC edit; in my opinion only) more likely to (PC edit; find success in an extremely stacked odds game of chance) than pull this off in a non pre-planned situation.
Another question was asked: "why wouldn't you keep the rear hand close to the face for protection?" Well, as I pointed out, I would! I'm wouldn't close inwith an extended Tan just for the heck of it! The Tan only goes out because a wide punch was detected AS the Wing Chun guy was punching. So the better question would be..."why wouldn't you KEEP your rear hand close to your face for protection?" Ever try to defend against a hard wide punch with just a cover ....like a boxer does....without gloves?
Well yes I have. I regularly spar without gloves. The best cover for a hook punch is a wu modification close to the head. It is fundamentally unsound to block incoming blows out away from the target in my experience.
That can be just as risky as using the Tan. As you note below, a powerful hit that can collapse a Tan could certainly blast through a high cover. And a collapsing high cover is much closer to your face than a collapsing Tan!
With all due respect for your undoubtedly well tested approach, covering your head (small radius) is easier than covering an arms length circle (large radius).