daemon - Writing and packaging system daemons

In the past days, I always confused why I need to activate a .socket daemon like systemd enable --now xxx.socket instead of .service.

Socket-Based Activation: In order to maximize the possible parallelization and robustness and simplify configuration and development, it is recommended for all new-style daemons that communicate via listening sockets to employ socket-based activation.

Ref: daemon(7) — Linux manual page