It's replacing one left-hander with another. Andrew Chafin was rock-solid in 2015, but had struggled badly in the early going of 2016. He appeared in 23 games, but had allowed 17 earned runs in 17 innings for an easily-calculated ERA of exactly nine. His strikeout rate was actually significantly higher this year, at 10.6 per nine IP, and he seemed to have been the victim of some very bad luck - his FIP was below three, but opponents' were BABIPing him to death, at a .457 rate, despite a line-drive rate virtually identical to career numbers (27% vs 26%). That wasn't enough to save Chafin, who was optioned to Reno on May 27.
There, the BABIP gods decided to take their foot off his neck, and he tossed 7.2 scoreless frames over five appearances, allowing only three hits and a walk with seven strikeouts. Zac Curtis hadn't been disastrous as a replacement, but had some control issues, with a K:BB ratio of 8:6. That played into Curtis allowing six earned runs in 10.1 innings for a 5.23 ERA, and the team seems to have decided Chafin was a better southpaw option out of the bullpen.