why there are box-pleats on men`s shirt?

Answer 1

The pleats serve two purposes. First it's a decorative element adding a bit of visual interest to the back of the shirt. Second, it adds a bit of extra "reach around" room for when a man has to lift his arms or reach for something; the fit is assuming that all men are physically active and need the easier fit. The pleats will give that ease and the shirt won't rip across the back from stress.