One point to true displacement is that some people use higher poly counts than you do. All the way to adaptive subdivision, which might make a vertex for every screen pixel.
Even at low poly counts, they're useful for low-resolution displacement. You don't need ten million vertices to turn a height map into low-poly terrain.
That doesn't mean they're useful to you. Blender has all sorts of tools that are only useful to some people. In years, I've never bothered to learn how to do camera tracking, for example.
Yes, you can use a displacement map to make a normal map. Simply use your displacement as the input to a displacement node in your material, then bake your normals. This is not the only way to make a normal map, nor is it the only thing one can do with a displacement map.
Displacement maps are useful as bump maps without baking as well, because they deform fine without relying on UV tangents, which allows you to use them on deforming meshes with arbitrary meshes and UV maps-- even without any UV maps at all.