1. A particular company will need the service infrequently
2. Many different companies will benefit from the service
3. The service requires a lot of manpower
4. The service is complicated and has a learning curve
So, to provide a concrete example, suppose you want to re-negotiate all your contracts with all your suppliers. This is the bread and butter of many large firms. However, you can't do it all the time, you only want to do it once every 10 years or so. It is quite complicated and requires a lot of people with experience doing it, and almost any company will benefit from doing it.
This is exactly when you would hire a consultant that is known and respected in this area.