To understand this, you have to understand that, no matter how much you want to re-invent the wheel, and try to think about an outfit that objectively will make a man look its best (disregarding of any message of wealth, sprezzatura, laid-backness or coolness he might want to portray with more trendy clothing) you will always arrive at the same point. A man will look his best, when he comes across as slender, tall, with broad shoulders (V-shaped), and his face, the most expressive, humane and important part of the body, in the foreground.
The way a beard (for men), makeup (for women), or a haircut can change the overall perception of your face and its proportions (although the face itself hasn’t really changed) and, thus, making you (appear) more better looking is the same way that clothes can change your body’s appearance without changing the body itself. The tuxedo, or dinner suit, is the modern product of decades and decades of fashion development, evolution, brain-storming, experimenting, and engineering of men’s clothing by the worlds savviest sartorial experts to arrive at an outfit that, per definition, will make a man look objectively its absolute best, independently of actual fashion trends. That is why it’s so classic and so timeless because it relies on pure objective beauty and not on what’s vogue. Every single detail has been thought of, and shouldn’t be changed as it will only mess with carefully engineered refinements.
It works in the following way:
The color black will make one’s figure appear even and, thus, slender and will give a mysterious appearance to the wearer (of course only in the evening/night, where black’s darkness is the richest). To make the man look broader in the shoulders, a V-shaped perception is induced through a big inverted triangle on the torso of the wearer. This is achieved by wearing a white shirt (for the contrast) and a one-buttoned jacket, to make the biggest and longest triangle possible. To accentuate this V, the lapels are covered with a contrasting material (silk, obviously) that accentuates the V-shape even more. To make a man look taller, an uninterrupted and even connection between the legs and the torso is sought, creating an elongated silhouette. This is achieved by covering the waist with a cummerbund or a waistcoat (low cut, otherwise it would break the triangle), avoiding, thus, the white shirt showing below the jacket’s button and breaking the ubiquitous darkness. The last part is to put the face in the foreground, which is done by creating contrast on the white triangle by a black (!) bow tie (and not a normal tie, which would break the triangle), which draws automatically the attention towards it, and imminently, towards the face (colorful bowties would distract the viewer away from the face and don’t create the same effect).
Basically, it comes down to the double V-shape of the lapels accentuating the torso. When fitted right it enhances a man’s posture and creates an illusion of a stronger chest and a slimmer waist.
Other factors contributing are the formal/uniform factor, nice fabrics with classics patterns and exclusive appearances, and that eye-catching examples of suits tend to be made to fit and accentuate all parts of the man’s body (shoulders, protruding bum, leg length, total length.)
A tuxedo makes a man look like a far more decent and respectable human being than maybe he really is.
Typically when a man is wearing a tux, he is also clean-shaven (or neatly trimmed) and has a fresh haircut.
A tuxedo also makes a man look more dashing, handsome, successful and confident.
Tuxes are usually only worn on special occasions, when people are looking (or wanting to look) their best.