Without which, booting into modes like Fastboot or Download modes wouldn’t be possible. Some times, flashing the wrong file can also potentially corrupt the Android bootloader itself. Generally if the device’s software is corrupted due to a wrong flash or any other software issue, it could be revived by flashing the firmware through Fastboot and Download modes.
The OEM flash tools can only communicate with a device and flash it through the said modes.
If you’re familiar with flashing firmware or custom binaries (like TWRP, root, etc), you’d know that it is required to boot the Android device into specific boot modes like Fastboot or Download Modes. Why and when would you need to use EDL Mode? When in this mode, the device identifies itself as “ Qualcomm HS-USB QDLoader 9008” over a USB connection. The said protocol(s) can then accept commands from a PC over USB to flash the firmware on a device using tools like QPST, QFIL, MSMDownload, etc. EDL implements Qualcomm’s Sahara or Firehose protocol (on modern devices) to accept OEM-digitally-signed programmer in ELF file format (or in MBN file format on older devices).