I don't use it since I CBA to learn another shell with a slightly different syntax but the original use case is: you're familiar with a Bourne-like shell and the associated Unix utilities but don't have it in Windows; I have Emacs so why not bring those tools with me wherever I go by writing all of them in elisp.
This is probably less relevant nowadays with wsl and stuff but oh well.
It does let you do some cool stuff tho like redirect stdout and stderr to a buffer; full tramp integration so you can go cp /ssh:user@host:~/whatever . or use cd to automagically ssh into the host; freely mix Lisp and shell code; a sane syntax for redirecting stderr to stdout; a crazy glob syntax; Plan 9's Rio window like scrolling. OTOH, it lacks job control iirc, pipes perform poorly and probably a dozen more but idk much.
(I at least have a function that translates TRAMP filenames into scp's filename syntax to reduce editing.)