Having a strong power play in the NHL isn't a necessity. The Boston Bruins and Chicago Blackhawks both had power plays that ranked in the bottom third of the league in 2013, but that didn't prevent them from reaching the Stanley Cup Final.
Still, an effective power play can make several positive impressions on a team. Outside of the obvious benefit of scoring more goals, a dangerous unit with the extra man can prevent the opposition from taking liberties during five-on-five play.
Tough Guy McGee is probably going to think twice about face washing your leading scorer after the whistle if the power play is clicking at 20 percent.
While reading this, please keep in mind that the percentage difference between the 25th-ranked power play and the 15th-ranked power play isn't massive. Typically, fractions of percentages separate teams within five or six spots of each other, so if you think a particular squad should be a bit higher, odds are it could be.
All power-play information appears courtesy of NHL.com.