Long struts with a small cross-section that have a compression load can buckle. If the job of the strut was to support that load without buckling, then you would want to do a buckling calculation to know if the design load is above or below the critical load. Increasing the cross-section can increase the critical load to a value much higher than the design load.
One problem with a linear buckling analysis is that it is not conservative. A perfectly straight strut with a perfectly centered load has the highest possible critical load. If the strut is not perfectly straight and the load is slightly eccentric, the critical buckling load is much lower. It's extra work to create deliberately imperfect geometry, but you have to do that to get a more realistic critical load. There is a method to do an initial buckling analysis then take a small percentage of the initial deformation into the nominal structure before starting a more conservative bucking analysis.