I like Affellay as well, but I'm not sure why someone would purchase him in January. (Cup tied)
Well, if PSV end up 4th in their CL group (meaning: no euro football for them this year) it'll be easier to get him. He won't mind not playing in Europe with Juve because he's not going to play in Europe with any other team anyway. Being cup-tied may mean that he'll be less expensive and yet less teams will be after him.
It adds to squad depth and if he's a quality player then that will help the coach rotate two quality players and rest one of them for CL.
Milan did the same with Ronaldo back in 2007 when they won CL. Ronaldo was cup tied and he couldn't play in CL but he was extremely helpful to Ancelotti because he was scoring in serie A and allowing Inzaghi the much needed rest for CL (plus, the risk Inzaghi to be injured was lowered that way).
If Milan didn't have Ronaldo back then and if Inzaghi was forced to play both serie A and CL, who knows what would've happened. It's very possible that Milan wouldn't have won CL and they'd have ended up 5th in serie A.