Being the sort of person who overthinks things, I have given a lot of thought about waistcoats. Why they are an important part of a fully dressed man's wardrobe, and how they can best be incorporated into an ensemble.
First, why wear a waistcoat? The answer if fairly simple -- it just helps to have a layer between your coat and your shirt and tie. The reason for this is that the shirt is, on it's own, an unshapely and drab garment, not intended to see too much of the light of day. It helps to have something other than a jacket to cover the shirt and add interest to the outfit This is the core logic of the waistcoat, and it can be broken down into several points:
- Waistcoats hide the shirt. Shirts are boring, and they should be boring. Too bold patterns in a shirt will unbalance an outfit by drawing the attention to the inner layer, rather than the outer layers. Likewise too vibrant or too dark colors are not a good idea for shirts -- the Regis Philbin look is an excellent illustration of this point. Thus a good shirt is a blank canvas, and it is dull and monotonous if too much of this shows -- particlarly when the coat is unbuttoned, or taken off. This calls for covering it with something that can have a more interesting color, fabric and cut. This helps especially when the jacket is unbuttoned or when a jacket is taken off, or when a bowtie is worn.**
- Waistcoats both secure and hide the tie. When a coat is unbuttoned ties tend to dangle and flap awkwardly. Witness our President in this photo:
Even with a tie clip or pin, there's still this long, pendulous strip of cloth just hanging down there. It needs to be -covered- as well as fastened down.
- Waistcoats shape the figure. Shirts, even tailored shirts, are not form-fitting garments. Their cut and material makes them unsuited to the task. A waistcoat, by contrast, is a form of mild waist-suppression that can subtly de-emphasize the waist and highlight the shoulders instead. This is particularly important when a man removes his jacket.
- They can mix things up. A 3-piece suit restricts wearers to wearing a vest that matches the pants and jacket, but that's hardly the only way to wear a waistcoat. A contrasting waistcoat, particularly one in between the shades of the shirt and jacket, can add a nice bit of 'texture' to an outfit.
*I use the British term to differentiate the waistcoat from other, less formal, types of vest.
**In the old days a man was undressed without his jacket, but that was because his shirt was his underwear. With the introduction of T-shirt undershirts and deoderant, there's no -hygenic- reason why men can't go around in their shirtsleeves.