I've never heard an explanation as to why Baristas will often froth milk in one pitcher, pour it into a second, and use the second for their competition's latte art. I continually want to improve my froth technique so I was wondering if I should begin doing this to get better art? But my bottom-line question is WHY do they do this. If I understand the "reason why" I'm far more likely to do it properly.

It's a quick way they can pour off the lighter, slightly coarser foam structure that floats to the top, without wasting it. If the chose to, they can re-incorporate a portion of it back into the original pitcher. Even with great technique, usually the milk on the bottom is denser, tighter bubble structured and makes greater latte art.

THAT makes perfect sense. Thanks so much!!!

When I make two single caps with a double spouted PF (which I think they do in competition) I've found it makes a huge difference. I froth a measured amount of milk for both cups in one pitcher. If I were to try to pour one, then the other from that one pitcher, one cup is too dense and the other too frothy. So I fast pour over 2/3 into a second pitcher, then pour the top of that back into the first, then do the first cup. Then pour the remainder back into the first to do the second cup. I haven't mastered it, but can get two single cappas with similar rosettes and little or no wasted milk.

To see someone who has mastered milk sharing between two or three pitchers, check out some Dritan Alsela videos, e.g.

Barista competitions are timed and each service usually includes 4 drinks...all while talking about the beans or the farm or how many children the farmer has or what the cow that produced the milk being used ate last week etc...milk sharing is a necessity to not draw out the service and burn time repeating one's self verbally and physically.

What Homeburrito said.