Being within 15 seconds of each other does not necessarily mean that the runners get help from each other for the orienteering or to keep up higher speed - but if two runners are within 15 seconds of each other in a large part of the course, it indicates that at least one of the runners has gained time due to the grouping.
These types of graphs are especially interesting in order to find out if a spreading method should have been applied in a race - and how spreading methods work if they have been applied.