I have an ESP32 development board and a NodeMCU board here. Both breaking out the pins for the SPI flash of the ESP. These pins are mentioned in standard pinout descriptions as must/should not connect. This pinout description states, that they
are not recommended for other uses. So, don’t use these pins in your projects
The answer to this question suggests, that you can somehow connect other SPI devices to the same bus as the SPI flash. The linked documentation from that answer states, that in that case you cannot control the chip select for the additional SPI device via a digital pin, but have to let the "hardware arbiter" (whatever that is) decide, when to connect to the additional SPI device.
That is the only use of these pins, that I could find, and even this seems pretty niche. It doesn't seem to be a standard way of usage for the pins and also rather difficult.
Considering the very limited and difficult use of the SPI Flash pins, why do ESP boards break these out to the user? Why aren't they just kept internal, making the board a little smaller?
Maybe there is another use of the pins or I just underestimate the importance of adding additional SPI devices to the bus.