Since Roissy runs a Game blog, he concentrates on dress within the context of pickup situations or explicitly social environments. Obviously, appropriate dress varies from place to place and with regards to an individual's intentions, e.g. impressing a boss vs. hanging out with friends. Clothes no doubt matter. We live in a shallow world. Or perhaps, more directly, we are a shallow species often content to judge individuals merely on the physical, a trait rationally derived from our evolutionary past. So dress does impact basically all aspects of life, cultural, social, professional, and sexual. It's an important way that we relay information about ourselves; though clothes do not make the man, an aphorism implying that facial features and body type can only be obscured so much. [Emphasis added.]
This is a rather incorrect way of looking at things. Shallowness is not simply an evolutionary trait; it is an economic reality. Humans are finite beings with limited amounts of resources, including the most precious one of all: time.
Clothes, among other things, serve as an indicator of status, in addition to serving as protection from the elements. This is one reason why dress is so important. Humans know that how you dress conveys a relatively large amount of information in a relatively short amount of time. Wearing designer label clothes is used to indicate wealth. Dressing like a gangbanger is done to indicate gang membership. Dressing in sweats shows the prioritization of comfort over status. Etc.
Though you can occasionally reach the wrong conclusion by judging on dress alone, you will be right often enough to make such methods of judgment valuable. And while it may be desirable that you use more than dress to make judgments of others, reality is such that this ideal is highly impractical, if not downright impossible.
Thus, as Roissy observed in the original post, clothes do, in fact, matter. In fact, they may matter more than Roissy originally concedes.* How you dress is a way to tell others about yourself without requiring them to invest a large amount of resources in acquiring that information. Thus, it is generally a good idea for you to dress your best.
* Although the context of Roissy’s original post is centered on Game, not general social settings.