Pleats are useful if you wear your trousers high, or if you are overweight. Otherwise they are useless.
Traditionally, most trousers had pleats rather than being flat fronted. Usually two on either side, and usually outward-facing (meaning that the hollow of the fold was on the side of the hips rather than the fly). The pleat closest to the fly was deeper. The English often wore their pleats inward-facing, considering the way they fell to be more elegant as less likely to gape.
But this was because they wore their trousers on the natural waist. This is easy to find: it is above your hip bones; often around the level of the belly button, though this can vary from one person to another. The waist is normally the slimmest point on a person, which is one reason men wore their trousers there, and why jackets are nipped in there – to accentuate the difference in width from shoulder to waist. It is certainly why women wore their skirts on the waist, and why many women still do so today. Their jackets are usually nipped in more obviously as well.
Now if your trousers have a high rise (the length from crotch to waistband) and are sitting on your waist, they need to go out before they go in. They need to expand from this slim point to get up and over the hip bones. Otherwise they would be skintight from the waist all the way down to the middle of the thighs. Pleats are necessary. They enable this expansion neatly and elegantly, dovetailing back into the line of the trousers when width is no longer needed after the mid-thigh point.
But you do not need them if you wear your trousers on your hips (as most do these days). This is also easy to determine – your trousers sit on the hip bone. The skin has hard hip bone underneath, not squidgy stomach. Why some men have trouble telling where their waist is I’ll never know.
Pleats worn on the hips make your thighs look like melons. They create needless volume. Combine pleats with a sharp narrowing to the ankle, and cuffs at the bottom of the trouser, and you have the abomination of so many American khakis. The pleats give volume at the top of the leg. The narrow ankle accentuates this proportion. And the cuffs make your leg two inches shorter.
It is hard to see how, without wearing pedal-pushers, a pair of trousers could be less flattering. Except that Americans have done it – they finish off the ensemble with a large pair of white trainers. So the trousers puddle at the ankle as well, their line being entirely unsuited to falling elegantly onto ankle-supporting Nikes.
Pleats, again only if worn on the waist, can be flattering for larger men. They mean that their trousers fall straight down rather than going in first, highlighting a belly.
That is the limited, albeit useful role of pleats. Ignore anyone who says they just want roomier pockets.