Because app manager doesn't work well. And there are the feedback on terminal that tell you about missing dependencies or broken packages...The fact you get those verbose log help for doing web research and solve lot problems. On GUI installing app isn't well done : it's slow, they don't tell you what they are doing nor why it fail.

The only limitation of terminal is when you want to work with file system. I need to see the tree and typing ls -a everytime isn't efficient. Example, i'm doing a git clone on a server throught ssh. But i have no way to know its structure and check if i downloaded it in the correct directory. I need a visual that tell me this folder is here, has those writing permission, is a tar archive... So i use both : filezilla and terminal, gui and cli. In fact, they are both very useful, so there no point comparing gui and cli, they both serve well their purpose.

I'm using CLI and GUI. For example, if i want to chose the correct keyboard and check its mapping : gui. If i want to add sources and its gpg key : app manager gui. There is no way i would enjoy typing this huge command line with flags from my mind, and i do lot mistype. Or installing the stack lamp ? on windows it was amazing and faster than linux. next, next, done.

Luckly we can copy-pasta those commands.

Edit : updated my text.