Both are, in general, used to control the area that's lit.
What are the relative advantages and disadvantages of both? When would you prefer one over the other?
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In general snoots give more gradual falloff than grids. Grids are effectively a collection of very tight snoots next to each other, as a result they spill much less light at the end and give you a more direct narrower beam. They are used when you really need to control the light to highlight an object/detail and restrict it to a specific area.
In addition to differences in the quality of light, shoots are much easier to manufacture in an emergency from objects you happen to have to hand!