A good coach can suggest different types of training that may allow the plateaued runner to climb upward to a new level of performance. A good coach keeps an athlete on course by making certain the athlete follows the system and plan, as above. He will have a check list of what's important about different phases of the training plan. So when it comes time to do a specific workout, the coach can remind the athlete what they are trying to achieve. This frees the athlete to concentrate on the actual training itself.
Most runners have a hard time evaluating their own training.
Keeping a diary helps, but still is no substitute for a good coach. Having a coach circumvents that, because a coach is an unbiased observer. A coach can look at your workload and evaluate it more objectively than the athlete. That's positive, because a coach can say your mileage looks pretty high, time to do faster work. Or too much speed, you need more of a mileage base.