If a user forgets their pattern lock or encounters a bootloop on a device with a locked bootloader, standard flashing is blocked. EDL mode is often used by service centers to flash a clean firmware image to restore functionality without requiring the bootloader to be unlocked.
Through EDL v2, advanced users can rewrite the modemst1 and modemst2 partitions, or even the NVRAM (non-volatile RAM) stored in the EFS partition—something fastboot cannot do.