In this tutorial you learn how to connect a layer to a path such that it can travel along the path or stay connected to a specific point on the path even if the path is animated. The expression we use can also set the rotation of the layer such that it follows the direction of the path.
This technique is very useful to link a path point or mask to a null or other layer.
What's the difference to Create Nulls from Path?
Have you used the Create Nulls from Path script to link shape points to nulls and had strange offsets between the shape and the nulls? That's because the Create Nulls from Path that is included in Ae does not deal properly with shapes nested inside transform groups (unless you keep all transforms at default value). In contrast to that, the Link to Point on Shape Expression we use here works fine with any transform groups.
Also, the expression allows to animate the point along the path instead of attaching just to a single path vertex.