Fashion designers are much more than the people who help design, sew, and make the clothes that we put on. While that might be the physical summary of their jobs, it actually goes way beyond that. The job of a fashion designer is much deeper than meets the eye (or in this case, the mind) and only those who are able to see beyond the ordinary will be able to grasp that.
Style is something that not everyone has. While most people learn are able to styles and are able to adopt them (which, in itself, is more like the easy part), some are actually born with the innate insight and the desire to set them. For instance, while the general populace can look at things like jumpsuit and ankle boots are the latest in styles and trends, they were actually thought up by someone. That person is the fashion designer: the custodian of style who is able to visualize a specific piece of clothing and make it come to life.
In truth, not all fashion designers have reached the world famous level of popularity, but one thing is sure; there is no fashion designer who is not in touch with style. The styles brought up by various fashion designers are usually based on a wide array of factors; backgrounds, experiences, personal preferences, what each person believes that fashion should be like, etc. These factors come together to influence what the person believes to be good style, and he or she is able to run with that concept in mind.
The substance part is where the physical comes in. Substance, apart from being the whole physical element of developing a particular piece or a fashion concept, also relates to the work that is put in; the effort that is put into making a piece. The conceived fashion styles might be the fuel, but there is a refinement process that goes into making these clothes look like what they are, most times, what fashion designers see and what they are able to think up are not what they end up making. This is because, in the design (or substance, if you will) stage, modifications and changes are made.
A fashion piece or trend is a product of the style the substance; both of which come together in the mind and surrounding of the fashion designer.