A TUI file explorer is a program that you can use to explore your file system and operate on files interactively in the terminal, without needing a graphical user interface (GUI) and without having to type long and risky shell commands (CLI).
Why would I use TUI over the GUI file explorers?
I think, if you have this question, most probably you don't need to use one, as they all come with varying degrees of learning curve. However, if you are a system administrator or a developer who needs to use the terminal a lot, you will find that most GUI programs don't play nice with your terminal programs. For example, you cannot or will not use your GUI file manager with your terminal based code editor like Vim, Neovim or Emacs, as that would be very inefficient, if not impossible.
Why would I use TUI over the CLI tools?
If you spend a significant part of your time typing shell commands and operating on files in the terminal, then you may need some kind of abstraction, i.e. automation over the commands you use the most. Using a tool that integrates well with your other terminal programs and is capable of providing real-time visual feedback, will make things very intuitive for you, and let you be more productive.
For example, if you only want to create a file with some text in it,
cat is enough. If you want edit functionalities,
vi should enable you to do so. If you spend most of the day writing code or editing text, you need something more powerful which integrates well with other tools. Like vim/neovim/emacs/vscode etc. If your development environment is more complex and requires a dedicated platform (like java/android ), heavy IDEs like eclipse / android studio will be needed. The same goes for file explorers.
Depending on your workflow and how much time you invest optimizing your development setup, you may find the TUI programs, specially the file explorers like xplr, very handy.
What I want to conclude is that TUI file explorer aren't there to replace the CLI tools or the GUI programs. Instead, they provide a framework for the users to utilize their favorite tools and programs to the fullest, using an intuitive and flexible interaction mechanism in the terminal.