Just installed Ubuntu and learning to use it, but can't understand something I'm trying to understand why people always tell me to learn to use the command line (or Terminal, in Ubuntu) to remove or install packages, etc, if I have the package manager. Also - what is the difference between the Terminal in Linux between the Command Line in Windows? I can't put my finger on a difference.
And one last question - I understood Linux is known for being tweaked a lot. But what ARE those tweaks? Are these themes, packages?

Thanks a lot!