Non-manifold is either harmless or bad in 3D computer graphics, depending on the situation.
Subsurface Scattering and Volume Shaders don’t like holes, for instance.
Booleans can give incorrect results.
The Dual Mesh Node (in Geometry Nodes) only works on manifold Meshes.
3D printing doesn’t do non-manifold Meshes.
But it’s not always bad; a Plane is also non-manifold!
