The IRAM is much faster, while the IROM is much slower and can only be accessed via a cache, which is also sometimes disabled, e.g. by the WiFi module or when writing to the flash. Therefore, time-critical code as well as code that has to work even when the cache is disabled must be placed in the IRAM.
During the write access to the SPI flash, the IROM cache is not available and only code from the IRAM can be executed. Therefore, the code of file system implementations which access the SPI flash must reside in IRAM.
ESP32 nodes can crash during SPI Flash write operations if required parts of the code are not in the IRAM but in the cached SPI Flash memory, which is disabled during the SPI Flash write operations. Therefore, the code of the SPIFFS package and the VFS module are now stored in the IRAM.
