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.
Consultants are also well suited to doing nasty jobs that have to be done right.
Writing the overview sections for proposals is an example. Also, porting software to new platforms, writing test suites, microcoding embedded software, writing device driver software are some examples that come to mind. What makes such jobs nasty is that most professionals would rather do something else instead. What jobs are not well-suited to consultants If you expect to need the skill for many years, the skill you need is obtainable in the regular job market, you have time to hire and train someone, and you expect to have challenging work that will keep the person sharp and happy, then you ought to hire a permanent employee to fill the job. It is especially unlikely that you will want a line management position filled by a consultant for more than a very short time.