You don’t need to use one of these apps. PieFed is perfectly capable of running in a browser, even a mobile browser. Also, you can install PieFed on your phone as a PWA (progressive web application) so that it can be run on your phone exactly like an app.

Some people prefer a different interface though, so these other apps provide options for them. Also, some people are making a transition from lemmy and these apps generally also support lemmy, so it makes the transition easier when the interface stays the same through the transition.