Look at the three links above the ROM chip. I suspect that they were to select for 16K, 32K, or 64K ROM chips, as the PAL was capable of controlling all three sizes. I wonder if the writes to select the unused banks were there as a form of copy protection?
Tursi and I found several of those when we were building the Multicart with the 8K ROMs way back when. I think he just NOP'd them out. Whether the writes to >6000->7FFF were there as copy protection, or just errant programming, who knows, but my money is on copyright protection, since DBT was very aware of the things in the market (like Gram Krackers) that could be used. Forget to write protect thing thing, and you have to reload the cart... Or, try to put their cart in a bank switched cart, and it would screw up the cart by switching to some other cart.