Fireplaces took a major leap forward in 1678 when Prince Rupert, nephew of Charles1, raised the fireplace grate.This increased airflow greatly improving the efficiency of the burning fire.
Fireplace grates provide many benefits and a hot burning fire is one. When the logs are elevated off the firebox floor, air is pulled from under the burning logs creating a draw. This causes a cycle. The draw of air burns the logs hotter and the hotter the logs burn the more it draws. This causes the fire to burn cleaner which means less pollution and less creosote in your chimney reducing the chance of a chimney fire. A hot fire also means more heat for you. When the fireplace grate is positioned towards the back of the firebox, the heat hits the slant of the fire back. The slant of the fire back slants forward reflecting the heat out into the room.
Another benefit of fireplace grates is they securely hold the burning logs in place so they are not rolling out onto your floor. This benefit should be obvious. Fireplace grates will also keep the burning logs compact which makes them "feed" off one another instead of having them spread out burning independently.
Fireplace grates also make lighting the fire easy. Simply crumple up newspaper and stuff it under the fireplace grate. Next, put kindling on the fireplace grate. On top of the kindling place medium size pieces of wood and on top of that place a log or two. Presto, a full size fire. The importance of the fireplace grate in all of this is to keep the weight of all the wood off the paper, which would smother it, not allowing it to get hot enough to ignite the kindling.
When measuring your firebox for a fireplace grate, simply measure the back of the firebox. This is usually the narrowest point if the colvings (sides) slant in. Make sure the is at least 2-4 inches of clearance on each side. The fireplace grate should have 6-8 inches clearance front to back to keep it as far back as possible away from the doors or screen.
Fireplace grates have a long history and are still as important today for many good reasons.