Novices concentrate on the "how" of something - Masters concentrate on the "why".

You see it martial arts training all the time.  Students in lines executing one movement or another, emulating the instructor.  The same thing is seen in the firearms world.  Lots of guys know "how" to - for example - reload the handgun, and they are spitting images of their instructor.  But unless they understand "why" it is important to do it that way, they don't really know anything other than to mimic what somebody else has taught them.  And it gets worse when that instructor didn't know either.

Just as Karate's moves have applications that must be learned and understood, the same must be said about the far simpler martial science of pistol fighting.

Often we will see somebody emulating a competition technique, or a technique that is workable only when you are a member of a team.  The reasons for this usually end up with - "it looks cool".  If that is the sum total of your goal in training - looking cool - or winning some game - then have at it.  Have at it because, even if you fail at that goal - looking cool - nothing will happen.  You may look uncool, have to endure a few snickers from your friends, but at the end of the day, you will still be alive.

If your objectives are more serious, such as preparing for and prevailing in a fight, then you must understand the "why".

"Why" does not begin with the technique, or the shooter himself.  Actually it begins with the fight, and the environment and circumstances of the scene. 

Why do we train a default to retain a depleted (but not empty magazine) when we conduct a proactive reload?

The "how" of our Proactive Manipulation Process will always be slower than the competitive based shooters, or the youtube heroes, but here is the why.  A single shooter facing an adversary will be alone, likely ambushed, likely facing more than one adversary, without back up or radio, or armor, or referee to blow a whistle to signify the end of the stage.  So we teach to shoot and keep shooting until the bad guys are dead, and you do not see anyone else to shoot.

If the pistol stops working during that process, we reactively branch to the Reactive Manipulation Process.  

But if that has not happened, and it appears that the fight is over - we accept that appearances may be mistaken - we move to cover and reload the weapon just in case the fight is not in fact over.  Irrelevant in a sporting event, it is imperative in a real fight.  And since you are alone, retaining rather than discarding your equipment makes sense.  Those three rounds left in your magazine may become a life changer if your perception about the end of the fight was totally wrong.

That is the "Why" the "How".

On board magazine is extracted and either inserted in a pocket or in the waistband.  We do not drop it on the ground for the reasons discussed...that magazine is valuable in that it may contain ammunition the lone shooter may need soon.  The replacement magazine is accessed and inserted in the pistol.  Once that is done, we charge the pistol to insure the round is chambered.  Remember that none of this requires a visual verification.  It is all done by feel and kept as simple as possible. We do one thing with one hand at one time. Our objectives are success in combat and don't care if it appears less cool.  That is the why. 

It is illustrated below.