Firebrick - Purpose and science

It's mostly for insulating the firebox to get the temp up to secondary burn temps and create that clean efficient burn.

Seems to me that firebrick is likely also part of the system of insulation (along with external shields) that allows the reduced clearances of many of the modern stoves. It also serves as the baffle in some stoves. Definitely serves to keep heat inside the firebox. In any case, it's an integral and important part of the stove as a system, and should be maintained in good shape. Rick

Replacing the cracked/broken firebrick will not significantly change your stove's performance, it is just part of good regular stove maintenance. The brick is thermal mass that helps the stove build and retain heat while protecting the metal from the most intense exposure. If the bricks are broken to the point that you are missing decent sized chunks, or the bricks are missing altogether, that's another story. The firebrick is integral to the design and normal operation of your stove and should always be in place as a simple matter of safety. Bricks with just cracks do not usually pose any danger. But firebrick is relatively cheap and easy to replace. If you need to cut bricks to fit a masonry blade that will fit a circular saw runs about $5 at the hardware store. Just make sure you cut outside -- this makes a lot of dust!