From https://github.com/puma/puma/blob/master/docs/systemd.md#socket-activation

systemd and puma also support socket activation, where systemd opens the listening socket(s) in advance and provides them to the puma master process on startup. Among other advantages, this keeps listening sockets open across puma restarts and achieves graceful restarts, including when upgraded puma, and is compatible with both clustered mode and application preload.

When using unix sockets, it should also increase security since only the webserver can connect to it.