An agency provides marketing services for individuals, but no technical support. The federal government and most states have rules about the distinctions between employees and consultants aimed at requirements for tax withholding, disability insurance and the like. Hiring through a corporation minimizes these problems, at a cost. Few technical consultants just give advice, although giving advice is one aspect of many jobs. In the technical world, most consulting service requires hard work writing, designing, and building.
Clients expect useful end products.
There are occasions when a relatively short answer to a specific question can be supplied by a consultant, who has paid the dues of long experience, but these occasions are the exception to the rule. Consultants are best suited to providing specialized skills not available from permanent staff. Sometimes the skills are in the organization, but just cannot be made available. A classic case is acquiring new business when top people are busy with critical ongoing work. Key people in the organization cannot be spared, but without submitting good proposals they will have far too much time to spare in the future.