Its not just DOMAIN knowledge.

It is also work routines and methods. I do a helluvalot of mentoring where ever I go. I try to help raise the overall knowledge in the business/organization to allow me to pursue greater improvements. Giving you my knowledge of "howto fix things" or "how to diagnose better" or "recognise patterns in chaos", means I can let others do easier work and lets me focus on really tedious and nasty work nobody else wants to do.

Companies just do not understand why they need someone like that, until they have one. Then they want one, until 90% of the crap work is done. Then dump them off at the local un-employent office once that 10% that will never get done is left.

Sometimes even fighting the unemployment.