The reason I use a term file manager is because it makes some cumbersome things much smoother. With nnn I can navigate directories at least as quickly as with a mouse in a GUI, can create new directories/files by pressing just two keys, and batch rename selected files in a Vim textbuffer. Also it's useful as file browser in Vim.

I don't use it for everything and I still copy/move files a lot without it. But it accelerates some things and is simple to use.