Attention, they want people to know that they are bad ass boxers.
"I don't lift all that heavy, but look at me shadowbox"
He then proceeds to start throwing punches while sporting his idea of what a mean, tough guy face, should look like.
yep, totally feeling small, so they gotta show how big they are. Same with the lame karate guys. Or even the Yoga dudes that gotta show how flexible they are in the middle of the free weight room.