Everyone understands the paradigm of double clicking an icon to launch the program or an installer. Many aren't going to understand the exact command they need to type into a terminal to get it to do what they want. This is almost always the answer for why people don't want to resort to using a terminal, discoverability. You can look it up, of course, but there's no good reason imo to not include GDebi for users unfamiliar with that workflow. Other distributions have it by default, but Cannonical is favoring their own tech here over the needs of the users. One more useless roadblock.

For me the struggle is: is the package called libcairo-devel, libcairo2-dev, cairo-dev or something else, i hate running a search everytime

My dad runs Ubuntu at home. He's completely computer-illiterate. When I need to help him install something over the phone, it's really nice to be able to have him download a .deb, double-click on it, and it just works. I've done this numerous times over the phone with him... it would take me an hour just to get him to type apt install properly.

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.

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.

It works marvelously for me. Never stumbled upon an issue with it.

This almost never actually worked in my experience. It was more of an annoyance than not even pretending to work, because I'd take time trying to get things to install in the UI and they wouldn't work before I eventually needed to revert to CLI.