The function of pleats is to create greater room around the tops of trousers, yet have that space collapse when not required, keeping the silhouette sharp and clean. They are particularly useful for those that use their pockets often, as this puts strain on the area. Arguably, they are also more useful on high-waisted trousers (those sitting above the hip bones, on the natural waist between hips and ribs) as the trouser has more work to do running from small waist to bigger hips, and then into the thigh. This is why RTW high-waisted trousers, for men and women, often come with pleats. Pleats also function better with high trousers worn with braces, because the braces attach just at the top of the pleats, pull on them, and so keep them under tension.
The downsides of pleats are that they can look a little old-fashioned – which is very cultural and subjective – and that they create an excess of unflattering material in the lap when seated. They are also generally seen as a little smarter than trousers without pleats (flat fronts) and therefore better suited to suits or formal trousers. There are several variations of pleat, in terms of number, direction and size. In general, the more there are, the greater the functional benefits, but also the greater the potential drawbacks.
Pleats can face in two directions: either backward facing, where the excess material is gathered towards the back of the trouser, nearer the pocket; and front facing, which is the opposite. See the different images above for illustration. Backward facing or reverse pleats are by far the most common on ready-to-wear trousers. Stylistically they probably look nicer, keeping the inward half of the trouser leg neater. Arguably front-facing pleats function better though, and some prefer the outer half of the trouser to be cleaner, between the pleat and the pocket. You can see that in the Igarashi image above: the right, outer half of the trouser leg is very neat, while the inner half is always going to be slightly messier. Historically, front-facing pleats have been more common in English tailoring, and backward-facing in Italian tailoring. The dominance of Italian fashion brands and manufacturers has been one reason the latter have become so much more popular.
Another variation is the kissing pleat, where the excess material is gathered between two sides of material. Although this might seem to be the best of both worlds – in terms of keeping both sides of the trouser leg clean – it does mean you lose the attractive sharpness of the pleat running down into the crease of the trouser. As such it is probably best thought of as a variety for its own sake. The other important thing to remember with pleats is that they work better in sharper, more densely woven materials. In general, trousers are made out of such materials anyway because it means they drape better and hold a crease. But not all are. Most worsted wools (standard suit material) are fine for example, as are linens. But some cottons and woollens are looser and softer – moleskin and most tweeds, for example. With trousers like these (chinos are another good example), any pleat might lose its sharpness and stay open, removing both its functional and aesthetic attractions.