Leaders are usually insecure. They need validation. Management consultants give it, for money. In theory, they can take the blame if things go very wrong. But in practice, management consultants tell you to do what everyone else does, and that means both leaders and consultants are saved by the "nobody ever got fired for buying IBM" effect.
But there's more to it than that. Your investors don't trust you if you're not the sort to trust Deloitte. (Your investors probably also invest in Deloitte, by the way.)
The value of validation from a consultant normally doesn't have anything to do with insecurity. Most corporate leaders got where they are because they are quite secure, even to a fault. But, corporate politics is a mess at a lot of large organizations. Bringing in an external opinion, is a helpful defense to politically motivated decision-making.
But there's more to it than that. Your investors don't trust you if you're not the sort to trust Deloitte. (Your investors probably also invest in Deloitte, by the way.)