[simonw]

Aside from the thing where it's useful to have an outside (expensive) influence to help make your decisions sound more confident than they actually are, a genuinely useful side of consulting is as a form of knowledge laundering between companies.

Let's say you're a supermarket chain. You have lots of problems that only a small number of other companies - your competitors! - also need to solve.

If you hire a big enough consulting company, they will have a large amount of internalized knowledge relevant to those problems that they gained through previous projects working with your competitors.

Of course, they're never going to deliberately reveal other company's internal secrets, or directly assign people to work with you who worked last week with your competition... but industry expertise and "best practices" still flow through these channels.