The warm-up period prior to each match allows teams to prepare for their upcoming competition. Please remember that this is not meant to afford individual players the opportunity to see if they can intimidate the competition by spiking the ball into their opponent's hitting lines. A similar courtesy applies to hitting out of the middle during warm-up.
Setters typically call out a warning prior to setting a ball in the middle.
It demonstrates poor sportsmanship and can definitely cause injury. It definitely does not prove that you are a great player. Great players demonstrate their abilities during the match. Fortunately, the number of offenders are relatively few. However, this reminder goes out to all players on all teams.