Even with systemd one may need to deal with pid files for services with more than one process.

Systemd has heuristics to detect the main process to send the kill signal or detect a crash, but it can guess wrong. Telling it the pid file location makes things reliable.

Plus creation of a pid file serves as a synchronization point that systemd uses to know that the process is ready. The latter is useful when the service does not support systemd native API for state transition notifications.

And the best thing is that even if systemd has to be told about pid files, one never needs to deal with stalled files or pid reuse. Systemd automatically removes them on process shutdown.