What would you rather do, play 4 minutes at even-strength, or get a 2 minute power play followed by a 2 minute penalty kill? If you had a normal team that didn't have an unusually dominant power play or penalty kill, then it likely wouldn't matter much in terms of winning or losing the game since you would not gain an advantage over the opposition in either scenario. If you are the goalie, though, you'd much rather play 5 on 5, because it means a lower chance of getting scored on.
Based on the numbers this season at Behind the Net, an average team that played all 60 minutes at even-strength would score about 2.4 goals per game. An average team that played 30 minutes on the power play and 30 minutes on the penalty kill would score about 3.7 goals per game, or about 50% more often. The scoring rate on the power play is roughly triple the even strength rate. The overall shots are likely to be fairly similar in both scenarios, the difference is that power play shots are much more likely to go in. Save percentages on the penalty kill usually hover around .870 or so, while the average save percentage at even-strength is up around .920.
Some teams appear to be more low-scoring than they actually are because they don't spend much time on the power play. The New Jersey Devils have been an example of this type of team, and another is the Minnesota Wild. The opposite is true for often-penalized teams like the Anaheim Ducks and the Philadelphia Flyers. A team that wants to improve its goals against without bringing in extra defensive talent can simply coach its players to take fewer penalties.
Teams that take few penalties tend to also draw fewer penalties, but a team that was terrific at drawing penalties while being also disciplined enough to avoid taking penalties would have a big advantage. That description fits this year's Carolina Hurricanes, who lead the league in power play opportunities for while also having faced the 5th fewest power play opportunities against. The Hurricanes special team units are pretty mediocre (Carolina has the 21st ranked power play and the 18th ranked penalty kill), so they haven't really taken advantage of this as much as they could have, but the team is still +4 in goal differential on special teams compared to -5 playing 5 on 5. If the Hurricanes had been exactly average in taking and drawing penalties, they would be expected to score 48 power play goals and allow 55, so the net result of their team discipline is +11 goals.
Combine team discipline with a weak division and some good play/luck in close games (the 'Canes are 19-7-6 this year in games decided by one goal), and you have a team that is in the playoff hunt. This just illustrates the potential impact and importance of penalties in today's NHL.