Great question, and the answer to it is simple - nowadays it is an art form, people love to purchase illustrations for their homes, they are used in fashion magazines, as well as communicating designs details to manufacturers - not so much anymore now that technology has advanced so much.
You can always google what it is right? From a fashion illustrators point of view you would have to ask a fashion illustrator. You're in luck because I am one. Long story short way back in the day people would need to illustrate looks for magazines to show people how to wear that seasons latest fashion, but as time progressed so did technology and photography became king in communicating fashion - making illustrations more of a art form and for me thats how I look at it.
Here are some examples below of my work as a fashion illustrator -