[vidarh]

You bank switch then. A common use would be to e.g. be able to use the ROM to load code into the RAM under the ROM and then bank switch, but also e.g. for extensions where you might want to first copy the ROM into the underlying ROM, and then patch whatever changes you wanted into before bank switching.

[flohofwoe]

And sometimes you also don't need to read the written data back (for instance when writing to video memory).