The Red Sox didn't necessarily need another right-handed reliever, but they have the space to acquire one at the moment, and a decent depth piece became available when the Braves designated Anthony Varvaro for assignment. The Sox plucked him from the Braves in exchange for cash and minor-league arm, Aaron Kurcz, who you might remember as part of the Theo Epstein compensation from 2012.
Varvaro, 30, spent the last four seasons with the Braves, but didn't become a full-time reliever for them in the majors until 2013. All told, he managed a 126 ERA+ over 168 innings, and while his strikeouts dipped to 5.3 per nine in 2013, they rebounded to over eight per nine in 2014. He's not an extreme ground ball pitcher, but he's capable of inducing them at close to a 50 percent rate in addition to his whiffs. He's a nifty sixth inning type who can come in and keep either lefties or righties from doing too much damage en route to the high-leverage relievers shutting things down completely.
Varvaro is not arbitration-eligible until 2016, and isn't a free agent until 2019. Who knows if he'll survive the offseason on the Red Sox, as they've been known to pick up intriguing relievers who have been designated then do the same thing themselves weeks later, but his presence could mean the Sox plan on keeping the likes of Anthony Ranaudo and Brandon Workman in a starting role at Triple-A just a little bit longer. That, or they just want to have options. Either strategy (or both!) would fit Ben Cherington.
As for Kurcz, he was Rule 5 eligible but not selected, and while he can miss a ton of bats, he misses the strike zone often as well. He'll be 24 in 2015 and is just getting to Triple-A now: that's not ancient by any means, but it's also not close enough to help the Red Sox right now as Varvaro could.