Monro,
This is a very good question! Thanks for asking it!
It is true that during economic downturns prospective clients will typically have less money, or more uncertain cash flows, and so they will be less likely to spend money on consulting services. As a result, consulting firms may earn less revenue during economic downturns, and some consultants may become unemployed.
Consultants and consulting firms can go some way towards reducing the effect of economic downturns by offering a range of services including some which are "countercyclical". That is, services for which demand demand increases during downturns, such as supply chain management, restructuring and turnaround support services.