Writing is rhetorical: it is designed speech.
You already use rhetorical choices in your day to day conversations because you interact with a variety of listeners. With those who know you best, you use insider jokes and a verbal shorthand based on your history together, but you wouldn’t speak to a tax auditor in the same way you do to your closest friend. Your boss cannot be spoken to with the same words you would use with a toddler. Your grandmother probably wants more warmth and familiarity from you than from the cashier at the gas station.
At work, around the proverbial water-cooler, whether that water-cooler is real or digital, you speak casually. Business conversations vary according to circumstance, though. 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 keep the precise subject vocabulary needed but drop the formal tone. With your boss? Part of being a good employee is learning when to adapt to whatever communication style the boss prefers. In all these cases, conversations are as cooperative, professional, friendly, or courteous as the situation warrants. Each time you think about your audience, then adjust what you say and how you say it, you are being rhetorical.
The shifts you make in various business conversations you need to attentively do in your business writings: weigh what you want to say against how your audience needs to hear it. Design your writing to suit your audience.