What worries you most about your business? Is it the consistency of advice? The communication of your brand? Legacy clients? Succession planning?
There are probably a multitude of areas that can be covered in house by staff and senior members. However, whether they have the time, the expertise or the drive to complete these tasks may not be the case. There is also the issue of 'internal' versus 'external' consultancy.
External offers an unbiased, 'fresh' set of eyes, with your goals in sight, and no hidden agendas.