Normals at neighbouring vertices have to be coplanar if you want the beams to be flat, but they do not need to be parallel.
It’s true that for any mesh you can set all the normals to be parallel, or all passing through a single point, in which case extruding the edges along these is just like extruding the whole mesh in one direction, or scaling about that point respectively.
However, for meshes with certain specific properties, non-trivial offsets exist, where the normals are not all the same or all through a single point. (See here for some more details : http://www.geometrie.tugraz.at/wallner/focal.pdf)
While it is generally possible to make such a mesh on a given smooth surface, you can’t choose the grid directions arbitrarily, as you generally need them to align with the surface principal curvature directions.
This also usually means the grid directions will not line up with the surface boundaries, so you can’t just constrain the mesh boundary points to the surface boundary, but have to let the mesh slide off the surface in some places. I shared some examples like this in these threads:
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