Grates are frames built out of cast iron for the purpose of holding fuel for a fire. They come in a variety of sizes and shapes depending on what is needed. Some of the benefits of a grates are they make it easier to light the fire. With the fuel being suspended above the flat surface, paper, a fire starter, or kindling can be lit below the fire to help get the larger pieces of fuel in the grate lit.
There are additional benefits of a cast iron grate. When a fire is burning the embers or ash fall through the grate and thus not smothering the fire. Grates make it so the the fireplace doesn't have to be cleaned as regularly. Since there is space between the grate and the base of the fireplace the ashes can be cleaned as need vs. after every fire without the use of a grate.