It focuses the eye on the line being read, and it really speeds them up. If they want to read faster, they should move the card faster. Don't mouth the words or try to pronounce them in your head. You can read faster than you can talk. Don't read function words.
Faster readers understand more.
It's hard to convince them of that, but if you say it enough, some of them believe you. If they keep reading, they usually begin to understand. Bring as much of your background knowledge as you can to the forefront of your mind. You can read things you know something about faster than what you know nothing about. These are not in any particular order.