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: