> If I write a server app that listens for client connections, why not daemonize that?
Because writing a daemon is almost always more work than writing an application that runs in the foreground. And systemd is capable of taking applications that run in the foreground and running them as daemons instead.
Put another way - if something else is doing all of the hard work for you, why would you want to write a daemon yourself, except possibly as a learning exercise?
Stated more accurately, services running under a supervision suite yield their daemon instantiation to the underlying process manager, for which enforcing a consistent daemon control framework is its purpose.
Because writing a daemon is almost always more work than writing an application that runs in the foreground. And systemd is capable of taking applications that run in the foreground and running them as daemons instead.
Put another way - if something else is doing all of the hard work for you, why would you want to write a daemon yourself, except possibly as a learning exercise?