Plain English writing is used to communicate in a business-to-consumer context. You have to take into consideration that not everyone understands the terminology and language that you would use because you are an expert writing about something in which you have a vast knowledge.  The reader may or may not be educated or knowledgeable in what you are writing about, so you have to write intentionally for them to understand you. This differs from writing in other avenues and in other contexts vastly. If you were to write a proposal for a marketing campaign that you would hand to your boss, you would be able to use all sorts of industry terminology because you can assume that they know what you are talking about, and they are educated in marketing. But if now you need to take that proposal to the company you are working for, the people there need to understand what it is that you are going to do, you need to write in plain English for them to grasp your writing. In marketing, our job is to effectively communicate with consumers, which makes plain English writing vital to success in this industry.