His bullpen has been successful this season, ranking fifth in MLB in ERA and WAR, but it has been top-heavy with Addision Reed, Jeurys Familia, and Hansel Robles each logging over 50 innings. A weak collection of middle relievers, spear-headed by the now-departed Antonio Bastardo, has resulted in more close games than there would be otherwise (though the weak offense is the main culprit) and Collins has found it difficult to give them days off when games are tight.
Bastardo's struggles also put a heavy burden on Jerry Blevins, the team's only dependable lefty. Blevins has been fantastic, with a 2.54 ERA and a 10.2 K/9, but he's racked up 49 appearances, many of them high-leverage. Josh Edgin, who was added to the roster Tuesday, hasn't pitched in the majors since undergoing Tommy John surgery in 2015, but he had a 2.57 ERA the previous two years and a 2.20 ERA in Triple-A this season. Edgin still walks too many batters and he won't see the pivotal game moments like Blevins. However, having a second lefty-specialist will give Collins the flexibility to sit Blevins in low-key games, plus bring in a specialist during multiple points in a game.
As unimpressed as people are with the return of Jon Niese, he does fill the role of long reliever, something the Mets have lacked completely since Logan Verrett was put in the rotation for Matt Harvey. No one should be happier to hear this news than Robles, who has pitched two or more innings in nine different games this season. Allowing Robles to transition back to full-time seventh inning role will keep him fresh and effective through the end of the season (and, hopefully, the postseason).
The big question is what the Mets will get out of Niese, who struggled so badly as a Pirate they traded him back to the Mets for basically nothing. The good news is that as long as he's kept away from the rotation, there's reason to believe he can fill a middle relief role effectively. He's seen some success in small samples, both this season and in the 2015 postseason. His struggles as a starter this season largely coincided with drops in velocity and his abandoning of various pitches, particularly his curveball. Shorter stints will allow him to throw harder and also get away with being a two-pitch pitcher.
Edgin and Niese are not game-changers, but each bring a different look that can help Collins shuffle around some innings and highlight the strengths of Blevins, Robles, Familia and Reed, all of whom will be needed during the final stretch of a playoff run.