Well there is another far more obvious use, I'm quite surprised no one mentioned it actually. Before I say anything I am quite aware that a chambered punch isn't a "fighting", "Sparring" or "Cage Fighting" technique!
However thats not to say with very minor mods it isn't a street friendly so called self defence technique which can be very very commonly used very very effectively.
for example during a barfight type situation(or any other just an example) before it "kicks off" during the whole posturing/talking/screaming/threatening stage, you will most likely be in close range with limited space movement, you may also not be able to get up any form of fence to aid you or not want to look overtly aggressive by putting your guard up as you may be able to talk the situation down.
however if you cannot one can't disagree that short of a nice kick in the groin or strike to a vital area (throat, eyes etc) a good hard straight punch to the chin/jaw will generally if done right put your oponent down pretty quickly.if the chambered punch from the hip is well practiced and one can get good snap speed and power from a punch launched at a relaxed hands down position, it becomes a rather powerful self defence tool for an opening either pre-emptive or reactionary defensive offensive strike.
just my 2 cents not saying that it is the best way to punch on the street or anything so please don't misunderstand me but it can work very well as an opening surprise counter if one is being threatened, and can therefore be used for what it is... a punch.