Traditionally, Chinese martial arts (of which Wing Chun Kuen is one) do not use “sashes” to denote student level, rank, or instructor experience. They don’t use anything.
Judo began the practice of using colored belts (initially a few, later more) to designate experience levels for competition. Philosophically, Judo was trying to move away from the older practice of Jujitsu and into a modern, sport-based structure. In martial arts, you know something or you don’t; you can use it or you can’t.
But the Judo idea quickly spread to Karate and other Japanese arts, and into Tae Kwon Do, American Kenpo, and other arts. Instructors realized that belts were a motivator for students (status being desirable) and a financial incentive for themselves (desire creating demand). White, brown, black became White, yellow, orange, blue, green, brown, black, became purples and reds and half-colors and stripes and all sorts of other ways to increase the steps, increase the motivation, and (for those who instituted belt fees, testing fees, association fees, and other surcharges) increase the profits.
When Chinese teachers came west and saw commercial Japanese and Korean schools with belt-ranking systems, some decided to implement a China-cized version, substituting sashes for belts. Others who had backgrounds in TKD or Karate and then learned a traditional, rank-less Chinese art, integrated sashes in as well.
In the best of cases (some would argue Judo still and Brazilian Jiu-Jitsu now both represents this), belts reflect skill level, aid students in improving, and cost next to nothing aside from the cloth. They can also be handy when schools become widespread, multi-state or multi-national organizations with set standards and high-mobility of students between locations. (Leung Ting’s WT and William Cheung’s TWC would be examples of this).
In the worst cases (some would argue McDojos are in part defined by this), the belts become something bought instead of skill earned (pay the fee, get the belt, actually knowledge and ability be damned), confuse students, and gouge them financially. (Plenty examples of this too, unfortunately).
I had or have colored belts in judo, karate, and BJJ, and did Wing Chun Kuen longer than any of those without any strip of cloth to denote it (traditional mainland sifu — never heard of the practice).
But that’s just it. Sometimes the practice of ranks and sashes can become so widespread, people actually refer to them without realizing how modern, and in some cases anachronistic, they are. The modern and commercial becomes seen as the traditional.
We gain the benefits of stability and interoperability, of scaling and fitting in, but what do we lose?
The small classes where the sifu knew every student and their individual level, replaced by the gigantic class with sub-teacher of the moment looking at colored strips of cloth before deciding which generic drill to begin.
Like most things, there are benefits and risks, and knowing both helps maximize the former and minimize the latter.
(Personally, I’m wearing a black belt right now — leather with a metal buckle from the mall)