I just had the case that indeed I HAD to specify my Android device ID.

My intention was to unlock the bootloader of my Android device.
But Ubuntu just did not find the device when in "Download mode" - as displayed on the device screen.<br>
When using like:

    fastboot devices

Had no output, just prompt.<br><br>
As soon as I used just

    fastboot -i 0x2e04 devices

it listed the device.<br><br>
For all my predecessor Androids on the same Ubuntu host optin "-i" wasn't necessary.

2e04 resp. 0x2e04 is the ID for Nokia. In my case Nokia 6.1 or 6 (2018).

Could be found with "lsusb" or "dmesg" like:

    saeufer@eckkneipe:~$ lsusb
    Bus 002 Device 002: ID 17ef:7214 Lenovo 
    Bus 002 Device 001: ID 1d6b:0003 Linux Foundation 3.0 root hub
    Bus 001 Device 014: ID 2e04:c026

Also mind "fastboot" needs root permissions.