We say and write what we observe. In the history, when an axisymmetric cross sectional beam (i.e. the beam's cross section still remains the same when rotated about its longitudinal centeroidal axis) was subjected to torsion, the cross sections of the beam remained plane/flat. This is true for both, solid beam and hollow beam. However, when a non-axisymmteric cross sectional beam (i.e. the beam's cross section DOESN'T remain the same when rotated about its logitudinal centeroidal axis) was subjected to torsion, it was observed that the cross sections didn't remain plane/flat. Infact, they wrapped when subjected to twisting. The general equation for calculating stresses due to pure torsion for a solid circular beam is shown below:
where T: Torque applied, r: distance from centroid, J: Polar moment of inertia. Now, the derivation of this equation involves an assumption that the plane section must remain plane, i.e. the cross section cannot warp. Therefore, this equation cannot be applied to any other non-axisymmetric cross sectional beams.
Warping basically refers to one half of cross section (above/below the neutral axis) being subjected to compression, and other half to tension. This behavior is what we usually see in a beam subjected to bending, however, this behavior is also observed in non-axisymmetric cross sections subjected to torsion only. Calculating shear stresses for a non-circular cross section is somewhat more complex and complicated than the circular ones.