The milled backplate can be various reasons but the only way to know what the intention was for sure is to ask the engine designer what his theory and take on that idea is. Some of the ideas already mentioned seem decent. It could be the volume theory, it could be the charge entry velocity and angle idea, it could be he wanted to save some weight on the engine not that the cavity in the backplate would do much for weight savings in my opinion but you never know or it could be a combination of all three or maybe something we haven't even thought of yet. I have run those engines with and without the cavity and honestly haven't seen much difference if any between the two.