While snoots can make a well-defined circle with a hard-edged transition from light to shadow, a grid's circle of light is softer at the edges. This can make the grid useful for subtler applications. For instance, as a background light in a portrait setup, the grid's circle of light is more conducive to the type of subtle transition from light to dark that works well in that situation.
When used on a key light, a grid won't change the quality of the light source—i.e., it will maintain a hard light's hard-edged qualities—but it will limit the spread and focus the light on a specific area of the frame. For instance, keeping the key light on a portrait subject's head, or in a larger-scale product or industrial photograph, a gridded key will put light where you want it without spilling into areas you don't. This ability to create separation in lighting makes grids a very powerful and versatile tool for any subject, in the studio as well as on location.
While a grid focuses light, it isn't as applicable for fighting spill and flare. Flags are still necessary with grids, just as they can sometimes be with snoots.
Fabric GridsWhat if you want focused light, but a softened light source? Well, there's actually a fabric grid option available for softboxes. The principle is the same, although the results are quite different. While a small honeycomb grid focuses light from a small source into a tight cylinder of hard-edged light, the egg crate-style grids that can be affixed to the front of a softbox will simply keep the light from spilling far beyond the primary axis of the light. In practice, that makes this type of fabric grid ideal for keeping a softbox key from spilling onto the background and changing its exposure. Fabric grids may not be glamorous and unique, but they're quite functional as a great way to maintain separation in lighting elements throughout a scene.
FresnelsSome lights have a built-in Fresnel (named for the inventor who first developed them for use in lighthouses) lens in the front of the fixture in order to adjust the spread of light from wide (flood) to narrow (spot). You may have seen Fresnel lenses in action if you've ever seen motion-picture lights, such as the fairly iconic Mole-Richardson hot lights (which helped to define the golden age of Hollywood, both in films and in the actors' iconic promotional portraits). They're also incorporated into many tungsten lights, like those from Photogenic and ARRI. ARRI's lineup of tungsten lights makes it easy to build a kit of constant lights from 150 watt output to 650, 1000, 2000 watts and more.
While Fresnels are most common in constant light sources, they're also available for some studio strobes. The Profoto MultiSpot has a removable frosted Fresnel lens that can be adjusted to focus the output between 10º and 40º, and an optional clear Fresnel is available, as well. The difference between a frosted Fresnel and a clear one is edge definition and falloff from light to shadow. A frosted lens will be slightly broader and softer; the clear lens will be specular and hard-edged. Speedotron offers the DeSisti 10-inch Fresnel Focusing Spot, which can output 4,800 watt-seconds of power, with a Fresnel that focuses the output as narrow as 10º and as broad as more than 50º.