,if a beam is undergoing axial forces as well as bending (due to shear force), then you have to take both the bending moment and axial forces into account. Bending happens due to equal and opposite forces act on a cross section with a certain distance between them in solid bodies, and when this force is multiplied with this distance, you get the bending moment at that cross section. Hence, at that cross section, the net axial force is still zero. However, if you additionally add an axial force to that solid body, then that can treated separately and stresses can found independently (like axial stress due to axial force and bending stress due to bending) and then they can be superimposed onto each other as long as the material still lies in the elastic range of the stress-strain curve. As you would have probably guessed, the net axial force in that cross section will still be just equal to the axial force that you added additionally, since the axial forces due to bending has already cancelled each other out. However, the effect due to the bending still remains.
This asserts that you have to always take bending moment into account when calculating Von-Mises stress (or in ANSYS, its called Equivalent stress). Since superimosing (i.e. adding the axial stress due to axial force and maximum bending stress due to bending) will result in sizing more sensible.