This tool will give you 11 evenly spaced points (but it's still called a 10 point divider for some reason) that can be set at any arbitrary distance between 1/8 inch to 3/4 inch. It stretches kinda like a really good Wile E. Coyote trap.
When laying out perspective grids, it's often the case that the vanishing point is off the page. Sometimes it's WAY off the page. What I used to do was tape down my art board. Then I would stick a little piece of tape on my desk way off to one side where my vanishing point should be. Then I would rule out a grid with my longest ruler. It was irritating and it sucked and I hated it.
A 10-point divider solves the problem by letting you just use the sides of the panel or the edge of the page to lay out a grid. It's quick and easy. You just mark points on one side, stretch or contract your 10-point divider and mark points on the other side. Then you can connect the dots.
It should be noted that you can do the same thing with a regular divider for about three bucks. But my 10 point divider is so cool that I don't even care. So I guess the point is don't buy one of these unless you want to play with something super overpriced and awesome.