If you have an actual eink Kobo, then you don't ever need the Desktop app. If you want to read epubs without DRM on anything other than eink you don't need it either.
It's not needed if buying ebooks from Kobo shop. The ADE isn't absolutely needed either though is the official way to put epubs with DRM on your Kobo.
Firmware can be downloaded here (which uses links to Kobo)
https://pgaskin.net/KoboStuff/kobofirmware.html
I never installed it even when I still used Windows.