It can also be really important things, like communication. In most jobs your team doesn't operate in a vacuum, you connect to numerous other teams and it's important to coordinate. When people are doing a job for 1-2 years then moving on, it's hard for those communication paths to remain stable. You can create all the documentation you want but when someone comes into any given job there is a lot to learn, and "when to let someone know you flipped a switch that affects them" or etc, usually isn't high on a priority list. When a lot of workers have been in the same role for 5+ years, they develop relationships, the communication paths become normalized and it gets easier to coordinate between departments, to make sure that changes benefit the company as a whole and not just one group. Maybe that doesn't make too much sense, it's one of those concepts that's easy to grasp once you've seen it, but hard to articulate.