Business writing is rhetorical: it is designed speech.
You already use rhetorical choices at work because you interact with a variety of listeners. In a conference presentation, you communicate with your audience using the subject-specific, technical vocabulary of your profession. In a project meeting with colleagues, you retain the precise subject vocabulary needed but drop the formal tone. Sellers adapt to the communication style their buyers prefer.
Each time you adjust what you say and how you say it, you are being rhetorical. Those shifts you’ve learned to make in business conversations you also need to do in your business writings: weigh what you need to say against how your audience needs to hear it.