Visual thinking is about communication. It's not about art. But sometimes, visual thinking and art do overlap a bit. Especially commercial art or graphic design.
Sometimes, whatever you're thinking about may not be so visual. Or you may not have time to draw something that truly conveys the message. Or it might take a more complex image to get the real meaning across. Or you may be working with others who didn't understand the picture you first drew. That's where lettering comes in.
Most people think visual thinking is all about pictures. And lettering doesn't really matter. But every mark on the page becomes part of the visual. That includes the letters! If you draw the letters, instead of just writing them like you usually do, they take on a whole new meaning. And look. They become as much of the message as the words themselves.
Whether your message is big and bold or soft and dainty, make it obvious with your letters!