Sure, a light with either a snoot or grid will create a circle of light when shot directly perpendicular to the background. The grid will create a solid circle of light with feathered edges and the snoot will create more of a radial gradient.
The effect looks cool and can also help to separate the shadows of the subject from the dark background.
The size of the circle can be controlled by the size of the grid/snoot and by managing the background lights distance from the background.
If you angle the gridded/snooted light at the background, it will create an ellipse, rather than a circle, so you can experiment with different angles get different different shapes as well.
Hope that helps.