This is a good move for the Phillies because they actually have some useful arms already in their pen. Jeanmar Gomez wasn't all that great as closer, but he's a better pitcher than his numbers suggest. Hector Neris was excellent, however. Youngster Edubray Ramos was solid, as well. Veteran David Hernandez was also solid, but is likely gone in free agency. It's likely that Neshek fills a hole that the Phillies would have otherwise needed to fill with a free agent.
Really, this move makes sense for both sides. The Phillies get cost certainty on a useful relief pitcher and the Astros get a bit of salary relief and a guy for their farm system.