More on seasoning. Steel tends to have a grain like wood in a linear fashion that gives it its great strength. Cast iron, while being able to withstand much higher temperatures than steel has a grain structure similar to a sugar cube. It does not flex well at all. While steel can be repeatably bent back and forth many times before breaking, cast iron can only take a VERY small deflection and will promptly crack if pushed or impacted beyond its limits. This is much worse with a "green" (new casting) piece of cast iron. The low fire used to season it allows the temperature to come up slowly and evenly to allow the grains to softly align themselves with their neighbors and retain its strength, allowing it to cool slowly enhances the process and the service life of the grate. A hard firing will heat it quickly and cause hot and cold spots in the length, every one of these creates a weakened area that is prone to fracture similar to the point of a weld on steel. The weld is strong and the steel is strong, the weak part is where they meet (this goes back to the heating and cooling). Once warmed slowly and evenly the grain is "set" and can take repeated high temperatures without developing the stressed areas it would on the first firing. Seasoning the grate is really normalizing the metal to make sure the grains have aligned themselves and are ready to take on some serious heat without fracturing.
The same process is used for cast racing engines, it keeps the machined surfaces truer (the bores stay round, the flats surfaces remain flat) and that makes more power and lives longer as the metal stops moving around once normalized. In the old days, you looked for a motor that Aunt Maude drove 40 short trips a day. Start it, stop it (heat and cool it at fairly low temps), over and over for years, when Maude was done with it after 10-20 years, you had a nice stable engine block. Today, the block goes in the oven for a day and gets a long, slow increase, then hold and then a slow decrease in temperature giving you the best casting for a engine block that will not move around after you've spent a fortune machining it right where you want it.