Depends, there are many ways of doing consultancy.

I do enterprise software consultancy, which means that those big companies that don't think twice about buying Oracle or Visual Studio Ultimate, hire companies like my employer to come in and develop software for them, instead of having an internal R&D department.

Usually because for those companies software is a by product, something that they need for their real business, the one that they earn money with.

Then you have business consultants, than come in, evaluate the current set of working processes and provide optimization guidelines based on a specific set of targets.

There are other variations.