Linux

I use dired in Emacs.

I assume you mean "why use these instead of file-manipulation commands in bash?"

I use both.

There are a handful of tasks that are easier in dired than bash.

• Making small modifications to filenames that aren't amenable to programmatic changes. You can just toggle the read-only flag on a dired buffer, edit the filenames, and then hit C-c C-c when done.

• Marking a set of files to perform an operation on where that set cannot trivially be expressed using tools in bash. Think, oh, "which movies do I like enough to want to keep around". This is especially handy when moving a number of files to another directory, which I think is why people often like the two-pane approach of orthodox file managers. Dired is not an OFM, but it can act like that if you have two dired windows open, using the other as the default target for the operation.

• Dealing with filenames containing obnoxious-to-type characters like weird Unicode stuff. If I want to delete the one file in a directory whose name consists of a bunch of kanji, it's easier to just manually select it in a list.

• Navigating where I usually want to see the contents of each directory. I'll often navigate around in dired while building up up an emms playlist. Browsing a list of movies to play.

EDIT: It's also not really a file manager, but I do use ncdu to see what's taking up space on a disk. I'll also use du -h|sort -h|less, but ncdu is, like file managers, more convenient when just browsing around the tree and looking at each as one does so, while manually selecting a few items to operate on (deleting).

EDIT2: I'll also add that virtually all of the people I know in person who love OFMs -- I'm in the US -- are from Eastern Europe, moved to the US from Russia, Ukraine, Romania, etc. I dunno why that is. Maybe just spreading along language lines. Maybe there are or were issues with switching between Cyrillic and Latin character stuff akin to my above irritation with kanji. But someone from Eastern Europe might have more input to answer your question.

EDIT3: The link I provided above for OFMs has a very long discussion from the author on why he likes OFMs (though not all terminal file managers are OFMs, many, like Midnight Commander, are). Reading it, I'd say that there's a lot of overlap with how Emacs works with dired+TRAMP+eshell and some other Emacs packages, though they accomplish similar goals in a different way -- sort of making integrated functionality that spans network file transfer, file management, text editing, file archive access, console commands, with a common toolset available for all. Would be quicker to learn an OFM than Emacs, though Emacs is gonna provide a considerably-larger set of functionality if you're willing to spend the time on it.

EDIT4: There are also a number of OFMs in Emacs, like Sunrise Commander, so I guess I shouldn't really treat it as an either-or matter.