In step 4 of the construction of a perpendicular line through a point why must the compass point be placed on the point where the arc intersects with the original line ?

In the diagram, points d and e are marked by drawing arcs of equal size centered at b such that the arcs intersect ba and bc. then, intersecting arcs of equal size are drawn centered at points d and e. point p is located at the intersection of these arcs. based on this construction, m , and m