This is statistic is used by football analysts to determine how many touchdowns a team allows per series, quarter, game, or season. It is used when analyzing a defense's ability to stop the offense from scoring touchdowns.
Having an easy statistic that tells you exactly how many touchdowns a team allows per game can tell you a lot about a team. While bad teams tend to give up a lot of touchdowns, winning teams can do the same thing. A high powered, high scoring offense like the 2010-11 Green Bay Packers can afford to give up a few more touchdowns because they score so much. A team like that, with a high winning percentage and higher than average number of touchdowns allowed per game, is a team whose game you will want to watch (and potentially put some money on).