In the gym we're constantly trying to get athletes to maintain good positions, whether it be during weightlifting, bodyweight/gymnastics movements, or even mono-structural conditioning elements (running, rowing, jumping, etc). The cues you hear of "chest up" or "chest out" or "set yourself big and proud" are really just tools we use to get athletes in better positions to move external objects or their own body through space. And really, posture is just a proper-sounding word for position.
Good posture in the gym hopefully leads to an increased awareness around better general posture in one's daily life outside of the gym. The folks at TedEd made this little video about the benefits of good posture - check it out:
WOD For 09-27-17:
2 Rounds For Time:
30 Deadlifts @ 155/105 lbs
30 Hand Release Push-ups
60 Walking Lunges
Medicine Ball Russian Twists (pick load)