We are so fluent in what we do. If yours is an arty biz that creates custom projects/pieces, chances are pretty good that your client/customer really doesn't quite know what they're purchasing---or what the heck you're talking about. They aren't fluent in the language of mural painting, mascot making, school performance, storytelling, whatever.
We use lotsa different fabrics, at AvantGarb, when we make mascots. Cavalier as I am, I thought everyone knew what lycra/spandex was. They don't. Now we call it a heavy-duty, stretchy, athletic fabric.
I'm not fluent in computery talk. All of the computer geek guys around AvantGarb try to speak in the simplest, most remedial terms to me. These are just a couple of examples about how we speak the same language, but don't understand each other.
...so when you think everybody has agreed on what will be done where, and for how much, it's a good idea to write it down . Your client may see something in the description and cost they thought they understood, but really don't understand.
Although we speak the same language, we don't all understand the nuances or each others WorkSpeak. A contract may not be poetry. Ideally, it's clear, concise as possible, and blunt.
Here's a really simple sample of a contract. If you forget something important, it will become an issue later. The point of a contact is that everybody understands what is expected.
In a contract we try to find each other's common language.