As described before, it's common on the c64 to have drive code in the computer and then send it over to the drive. Given that the drive code is executed in the other location, it's handy to use the region feature.

Once the code is sent, it's normally so that the area is available for other purposes. In the case at hand, the place is the location for bitmap which is going to be manipulated. There is a table that point to different parts of the bitmap (one table of lowbytes and one table of highbytes). Normally 6502 bench handle them beautifully, but it seems the region definition messes things up. It seems not possible to have the table point at the address occupied by a region.