There are three main types of consultants that small businesses use;
1. Strategy consultants... these people concentrate on getting all the great ideas from your head, mixed with some expert and advise and and some reality of the world to help you work out WHAT TO DO NEXT. This is especially importrant for a rapidly growing business as often you can be busy fighting fires rather than staying focused on the long term so that decisions you make today, will be relevant tomorrow.
2. Implementation consultants help you ACTUALLY DO IT. They need specific expertise and experience in a related area and can really help with the diverse range of things a business owner must manage. This sometimes the "doctor syndrome" or a feeling of inadequacy that you can't do it yourself can creep in. In really its about getting the job done.
3. Systems consultants similar to strategy consultants are more focused on tomorrow rather than today; but rather than dealing with the dynamics of the business, concentrate on streamlining the day to day operation. You can make the business more "scalable" by working on systems that maintain quality and allow to manage the business as it grows.
In an ideal world you would use outside people for all of these tasks to ensure you get the best expertise.