It is a reasonable question for a potential client to question the need for a consultant. One important benefit of using a consultant is that he or she comes into the organization as an advisor, negotiator, facilitator and often an industry expert. An outside consultant offers objectivity that can sometimes be difficult to find among internal stakeholders. A consultant does not have an emotional attachment to decision making for the organization, offering an objective, outside perspecitive which may help the organization optimize its decisions.
The client organization may not have the in-house expertise to develop and implement all aspects of a marketing strategy. The consultant provides the additional manpower and resource, bringing a specific set of skills and expertise. The consultant also has the benefit of experience in similar situations which may help avoid pitfalls and costly mistakes. The client is not paying for the consultant to "get up to speed".
And finally, sometimes a business needs validation of its plans and strategies for external parties such as financial resources and regulatory agencies. The consultant brings that external expertise and credibility to the table.