I believe its original motivation was just speeding up boot times by starting fewer services, even if you'd eventually want the service running. This was achieved in the past with xinetd, but systemd made the approach more popular for the masses.
replyinetd began to fall out of favour in the mid-late 90s as services became more heavyweight and startup times became longer (think of the initial crypto setup needed by sshd vs rsh/telnetd)
replyCPU speeds have increased & and i/o latency has decreased so much since then that startup times are generally imperceptible, so the pendulum has swung back to favouring socket activation.
The anti-systemd "traditionalists" never seem to acknowledge that history, though!