Just something about knowing a person is an amazing and knowledgeable resource and he’s choosing to work here of all places is a subtle morale boost. When they decide to leave, others might start asking “well if that guy left, am I still making the right decision to stay?”
I’m sure we’ve all experienced someone important leaving our company and having a waterfall of turnover that follows it. That is very disruptive and more should have been done to keep that one employee or at least keep the rest that followed them out the door.