Visual thinking is all about using visuals to help you think better. And if you're going to think about something, it's got to hold your attention. Maybe even grab it in the first place.
Great big blocks of text can be pretty boring visually. And if you're not interested in the content, your mind will probably wander. Probably far, far away from the content, too. And that doesn't help you get stuff done.
If the text is a bit more visually interesting, though, it will hold your attention. When there's stuff to look at, people will look! And if they're looking at words, they'll read them, too. Even if they're not interested in the content. The visual appeal draws them in.