What is the purpose of wing stiffeners?
When you have a thin sheet that is put under a shearing load like a spar web as shown, the sheet, being flexible, has very little resistance to bending under compression out of its surface plane (or buckling) at which point its ability to resist the shear force is lost.  The stiffener just gives the flat web some resistance to compression buckling by making it less flexible; "stiffening" it in other words.