He clearly offers something valuable off the pitch in terms of professionalism, his contribution to training etc. We only see a tiny fraction of the working lives of these players. The professional culture for the other 99% of the time will have a huge impact on success on the pitch. Anyone who runs any kind of team (or, hell, works in any kind of team and has their eyes open) knows how much this stuff matters.
He'll be on a cheap, short contract presumably. Meanwhile, he didn't actually play badly this season, but also certainly won't be impacting on their desire to sign one or two top CBs - no one in their right mind considers this a solution to an actual gap in the squad.
Seems like a no-brainer to me.