Retopology option: project individual elements onto the surface of other objects

What does the option “project individual elements onto the surface of other objects” do in the latest versions of Blender?

I’ve written tutorials on retopology in the past (e.g. using “Snap during transform” mode [horseshoe magnet]) to extrude-model a low poly mesh, edge by edge, around the surface of a high poly one. From memory the “project individual elements onto…” option was necessary because if you did not activate it, the new low poly mesh would not see the high poly mesh and therefore would not retopo around it.

Now when I play with it, retopology seems to work regardless of whether the “project individual elements onto the surface of other objects” button is pressed or not. It is as if the devs decided to make it work whenever snap mode is active, perhaps forgetting to just delete the GUI button altogether.

Does the button actually have a purpose? I seem to remember it as being a necessity, but now it seems redundant… or am I missing something here?

It projects all verts/edges/faces onto the base object if you have multiple selected. Without it, only the last selected element is projected and the others simply align to it.