Why do you use a grid on a flash?

A grid softens the light somewhat, an effect I really like. But the main use for a grid is to avoid the light going everywhere.

Look at this image: lit from the side with a bare speedlight (a Canon 430EX, which is equivalent to a Nikon SB600) with a red gel.

As you see, light hits the wall.

Now look what happens when I put a 1/4″ Honl Photo grid on the flash, with a gel on top of the grid.

Ah. No more light spill onto the wall.

That’s all. As simple as that.

And here’s what a Honl grid looks like: