Many systems will have this CSR as only readable (along the lines of my earlier P.S.).  But flexibility is allowed for possible use cases where the pointer wants to be changeable by M-mode to point to another memory-resident config structure.

Pulling a theoretical possibility out of the air - which is a form of bootstrapping scenario:  Maybe a tiny config structure in a small on-chip or off-chip ROM/PROM is read to find out some key initial system characteristics - including where to find the full-blown structure.  And maybe that structure first needs to be read into memory from an off-chip SPI flash device.  (Maybe both the full config structure and the associated decoder/parser code blob need to be read in.)

Or maybe this is a single-stage scenario where the config structure and code blob need to be read in from SPI flash by the early implementation-specific boot flow and placed in memory - and mconfigptr then set to point to the config structure.

The point is to allow flexibility and not foreclose useful scenarios that we may not readily imagine or expect at this moment.