Oh yeah, used to be at least two more ways. Not sure which one of these are not possible but anymore, but you used to be able to do:
1) Synaptic package manager which is a GUI for searching and installing .debs. My first exposure to a package manager when I first installed Ubuntu some 12 years ago.
2) Simply double-clicking on the file, opening the Gdebi (I think?) UI for installing that specific .deb file.
In my experience, simply double clicking on the file never actually worked for installing debs. The UI would pop up, give me a nice pleasant "Install" button, but clicking it would either no-op, crash the UI, show a progress bar that stalled out, or act like everything worked perfectly but not actually install it. Command line install works flawlessly every time.
Judging by your other comment (https://news.ycombinator.com/item?id=22954061) you were using Software Center for the installing. Gdebi was working fine back before Software Center.
I was using whatever popped up in a default Ubuntu VM when I double clicked a .deb file. If they've chosen to make that not the best software for installing .deb's... lets just say I can see why people say Linux distributions tend to be user-unfreindly.
Ok, great story, but not really relevant to the conversation around the difference ways you can install a .deb, which is the subject of this particular sub-thread.
Synaptic should still work at least for the X session. I haven't used it in a while, but it is a really good package manager. They got it right a decade ago.
1) Synaptic package manager which is a GUI for searching and installing .debs. My first exposure to a package manager when I first installed Ubuntu some 12 years ago.
2) Simply double-clicking on the file, opening the Gdebi (I think?) UI for installing that specific .deb file.