Each camera manufacturer implements their own proprietary automatic through-the-lens (TTL) flash metering and output control system. Unlike with manually-controlled flash where you set the flash power, TTL allows you to trust the camera to determine flash output for your portraits. This is useful in some situations, and not particularly necessary, or useful in others.
I suggest that you have TTL-capable flash units available even if you mostly use manual techniques, as I do, for flash portraiture. You never know when TTL might come in handy, and these flash units are as affordable as manual-only units.
Setting up TTL remote flash triggering using a compatible external TTL flash trigger: