A court motion is persuasive. But so is a letter to a client: there’s the inherent persuasive element when you convince your client of a point, but the quality of the writing itself also demonstrates that you're thorough, credible, and trustworthy. The same goes for a transactional document: if it's slipshod, readers will question your accuracy and attention to detail. Even something as simple as an attorney’s biography on a firm’s website is persuasive. While it’s just a set of facts, it persuades people of your expertise.
That's why everything you write is persuasive: clear and concise writing persuades your reader that you are credible and authoritative.