As with any item, a company usually develops a product and carries it based on high volume of sales and/or the request of its' customers. That said, I can actually think of practical reasons for that rig.
1. You possess a mesomorph frame and have no problem hiding your EDC and BUG in the same holster.
2. You drive professionally and find waist carry uncomfortable or a safety hazard (back injury or paralyzation in an accident from hip/SOB carry) and wish to carry a BUG.
3. You have a bad back or are confined to a wheelchair and must carry the guns above the waist.
4. You have arthritis, Parkinson's or nerve damage that makes reloading the magazine under stress a chore, but you are a competent shooter.
5. You are truly ambidextrous.
6. You want to improve your off-hand shooting, so you carry and draw from a mirrored rig.
7. You ride a motorcycle and your shirt is constantly being loosened and flapping in the wind, causing a brandishing situation in some states if your EDC/BUG is on your hip. (This problem has been expressed to me from other riders)
Just the subset of professional drivers (Truck, delivery, taxi, etc.) is in the hundreds of thousands, so the base is big enough to carry the product, not to mention the film buffs added to that that want to be just like Bruce Willis!
“Malo periculosam, libertatem quam quietam servitutem." (I prefer the tumult of liberty to the quiet of servitude.). Thomas Jefferson
"All that is necessary for the triumph of evil is that good men do nothing."
-Edmund Burke, Loosely translated from Thoughts on the Cause of Present Discontents. (1770)