Binding spline vertices via the Refine and Bind functions in Editable Spline (Vertex) is useful for connecting splines when building a spline network for use with the Surface Modifier.
Bound vertices are black, allowing them to be easily distinguished from standard vertices. You cannot move a bound vertex directly. However, changing the segment length by scaling it or by moving a connected vertex causes the bound vertex to shift its position in order to remain at the segment's midpoint. You can change a bound vertex's type (right-click the vertex, and then choose the type from the bottom of the right-click menu), and if you convert it to a Bezier or Bezier corner, you can manipulate its handles. See Connect.