Changes the existing polygon component selection to the vertex faces that are associated with the polygon components originally selected.
For example, if you have a polygon mesh comprised of four-sided polygons (quads) with two faces selected and then choose Select > Convert Selection > To Vertex Faces, the selection is converted to the vertex faces associated with the two previously selected faces. That is, eight vertex faces become selected based on the two previously selected polygon faces.
Vertex faces are used when assigning color per vertex information to polygon faces. You can assign a color to each vertex associated with a face. Even if a vertex is shared between faces, it has a color for each face that shares it. The colors at the vertices of a face are blended together across the face.
Many of the convert selection features are available from the context sensitive polygon marking menu when you press 
+ 
whenever a polygon component is selected.