Coming into this season, it was very difficult to figure out just how good the Braves' defense would be. On the one hand, they had two players who were inarguably the best at their position, in Andrelton Simmons and Jason Heyward. And they had BJ Upton, who was also near the top. Then they have a lot of guys who are somewhere between average (Freddie Freeman) and terrible (Chris Johnson). How exactly this was going to shake out was really anybody's guess. Further, Jason Heyward was injured for a large stretch of the season, meaning we got to see Evan Gattis play the outfield a decent amount.
Now that Heyward has been back from injury for a decent amount of time, I thought we could take this opportunity to re-examine the Braves' defense.
The first and foremost thing you look for in a team defense is simply converting contact in the field of play into outs. We can primarily see this simply through the lens of BABIP allowed. The league average for BABIP allowed is .299. We find the Braves are 2nd in MLB to the Pirates at .279 (the Pirates are at .273). In a slightly oversimplified way of thinking, this means that the Braves are essentially turning .300 hitters into .280 hitters, with defense alone. Given the sample size (1638 PA), it's difficult to say that this is due to luck. Whereas a hitter by himself might run into bad BABIP luck, for a defense, over a sample that large, it's a fairly reliable indicator of the true aggregate ability level of the team.
Next, we should consider the number of errors made. The numbers above exclude errors from calculation, as they're better dealt with separately. The Braves are 13th in fielding percentage and 14th in total errors made. Obviously, given a 30 team league that's very middling. The issue then becomes how do you weight these two aspects of the game in relative importance. Historically, errors and fielding percentage were often considered to be the be all and end all to defensive evaluation, but let's consider the question a bit more thoroughly. First, not all errors cause an out to not be made. That is, some errors just allow a player to advance an extra base. This is substantially less important than an error that allows a hitter to reach base than when he otherwise wouldn't. Secondly, the difference in outs made by the Braves' BABIP allowed from an average team over the same number of PAs actually overshadows the number of errors the best team in the league made, by a substantial margin anyway. An average team with a .299 BABIP allowed would allow 490 hits given the 1,638 balls in play against the Braves. The Braves have allowed only 457, for a difference of 33. To put that into perspective, the difference between the Braves errors (36) and the team with the fewest errors (the Orioles with 20) is less than half the additional hits they prevent compared to a league average team, and preventing hits is more valuable anyway. So while errors are important, they're often wildly overvalued. While the Braves are a middling team when you just consider fielding percentage, they're elite when you consider the more important statistic of BABIP allowed.
Another thing to consider is how well the team prevents extra base hits. That is, playing shallow in the outfield and catching a bunch of would be singles could theoretically lower your BABIP allowed, but in exchange for giving up a lot more doubles. Luckily, we have a stat that perfectly weights the relative values of these types of hits. wOBA. By looking at wOBA allowed on balls in play, we can get a true idea of the relative value of the Braves' defense. We see the Braves again are in second place (.273) to the Pirates (.264). The league average is .293. When we take this difference in wOBA allowed and compare it to the league average, we get that the Braves have prevented about 30 runs, compared to a league average team, with their defense. This number slightly eeked out the Pirates for first, by .7 runs (since the Pirates have had fewer balls put in play against them than the Braves).
This last bit brings up another important point, the Braves rely on their defense amongst the most in the league. When you add homers, walks and strikeouts together, the Braves pitching staff is 4th fewest in MLB, behind the Nationals, Mets and Twins. The Braves have 661 plate appearances against them that resulted in a walk, strikeout or homerun, compared to a league average of 730. On the flip side the Braves are slightly above average in number of PA that the defense came into play (single, double, triple or ROE). That is, it seems that this team has been put together with a very sound organizational plan, to field an incredible defense, and then use pitchers who leverage that, by having the ball put in play a lot.
When putting together a team, we have to consider the most cost effective way to do that, especially for a cash strapped team like the Braves. While ideally having elite pitchers who strike a lot of guys out and don't walk people or give up homeruns would be the best answer, such pitchers tend to be hard to obtain and expensive even if you can get them. On the other hand, having a perfectly married defense and pitching staff, that leverage each others' relative strengths so well, may be much more cost effective. For while Justin Verlander is incredible, and given the TIgers' defense, maybe the Tigers need a guy like Verlander, they're forced to pay outrageous sums of money, precisely because the have to have a guy like that in order to get outs. This marriage of defense to pitching is what has allowed the Braves to be 23rd in MLB in strikeout rate, but 2nd in ERA. The Braves' pitchers don't strikeout a lot of guys, but they also don't walk very many (2nd fewest walks) and the don't give up a lot of homeruns (6th fewest) and when teams put the ball in play, the Braves do an excellent job at converting those balls in play to outs.
You might say the organizational strategy of pitching and defense fit one another like a hand and a glove.