Team defense is an important contributor to wins and losses. But defense seldom gets the credit it deserves because direct measures of runs allowed are usually assigned to the pitcher, either in the form of RA/9 or ERA. However, suppressing runs allowed should be viewed as a run prevention function, which is jointly produced by both the pitcher and fielders.
Since RA/9 is based on the combination of pitching and defense, analysts try to sort out the contributions of pitching and defense by using Fielding Independent Pitching (FIP) to valuate pitchers or deducting the estimated impact of fielding to derive a pitcher’s bWAR.