An efficient implementation begins with a solid plan, taking into account those things you need to work on, leaving out those things which are already in place, and developing an accurate estimate of how long each implementation phase should take.
To help interpret the standard
A consultant who understands the standard's requirements can prevent wasted time doing things the standard does not require, or doing things in a way that does not meet the standard. You do not want to have to undo any of your hard work.
To allow you to benefit from experience
Using a consultant allows you to begin work right away without having to learn things on your own, and without having to learn by your mistakes.
To watch your timeline
A consultant can work with your steering team and ISO point teams and make sure the work is done within the time allowed on the timeline.