they get created when updateModelBounds is called. Internal implementations deal with this for you, as you should with your custom mesh/geometry.
for example the internal shapes (box, sphere, torus) do this after generation.
you should call updateMeshBounds and updateGeometryBounds whenever you change the size of it. usually when you edit it. you should generally know when it’s likely.
one might make their own for a collision shape around a camera. a boundingsphere i could use to check for collisions with.