The Shrinkwrap Modifier… is…

…A Hard-Surface Modeller’s Best Friend:

In its simplest form, the Shrinkwrap modifier is tasked with snapping the current object onto the surface of another object. It also has the ability to only snap specific vertices if you specify a vertex group… I think the Shrinkwrap modifier was probably first created as a retopology tool, the snapping allowing you to easily create new, low-poly geometry, over your high-res model, without having to constantly think about manual snapping. (Link to Article)