Consulting companies built their reputations on their work in the 60s, 70s, and 80s. They encouraged companies to do things we now consider Business 101 (it was cutting-edge stuff back then though). Several major companies in the 1960s had no insight into their competitors or even a full picture of the market they were in. These were the sort of organizations that collected very little data beyond sales and balance sheet figures - it was like they lived in a "bubble". The pioneering consulting firms popped those bubbles and actually taught businesses how to do basic market analysis, and turn that analysis into business strategy.

If you're at all curious, I'd recommend reading The Lords of Strategy by Walter Keichel, which a (albeit opinionated) history of the consulting practice.