While it is not necessary to view bending moments, it can be useful, especially for irregular structures. Even though principal moments are important, the default moment contours plans are for Mx (moment about the x-axis) and My. This is because most designers detail reinforcement orthogonally, and the directions are usually the x- and y-axes. You can view moments about any axes, including the principal axes.

It is not particularly easy to assess the moment contours. This is why Plot Distribution Tools are so useful.