Institutional knowledge is everything a long-tenured person knows that isn't written down anywhere. Why a system was built the way it was. Which vendor will quietly ship on time and which will miss every deadline. The specific reason a config flag exists that nobody has touched in eight years. The political map of who actually approves what versus who's on the org chart.
It's the most expensive thing to lose and the hardest thing to transfer. When companies do layoffs by salary band, they almost always destroy institutional knowledge as a side effect — and then spend the next two years rediscovering decisions the laid-off people had already made for good reasons.
For a senior consultant, your own institutional knowledge is the asset you're actually selling. Not the deliverables — anyone can produce a deliverable. What clients pay a premium for is the judgement that says "don't do that, I've watched three companies try it and here's what broke." Documenting your own institutional knowledge into reusable frameworks is how you turn twenty years of pattern-matching into something you can sell more than once.