Back in the day there were manual flash’s were simple. An electrical contact inside the camera closed a wired connection with a battery pack in the flash handle which then sent a jolt of current through a glass bulb stuffed with magnesium filaments. A blast of light ensued that would have you seeing a giant blue dot in your field of vision for five minutes and was hot enough to make the glass bubble. You would set the exposure on the camera.
Flash bulbs were replaced by flash tubes and eventually the camera and flash units learned to talk to one another across the hot shoe. The flash could “see” the scene through the camera lens and the flash power and duration were set automatically. It wasn’t long before someone came up with a connector cord that let you put the flash on a bracket handle with a diffuser.
One advantage wires had was the ability to act as an extension of the hot shoe. The camera and flash could still work together, albeit thorough a coiled cord connecting them. The camera could tell the flash the proper exposure and flash power, just as if the flash was reading the scene through the camera lens.
The wires connecting the camera to the flash unit were eventually replaced with wireless transmitters and that’s where the confusion began. Companies like Canon had their own proprietary wireless e-TTL technologies and, for a long time, no one could figure out how to get that to work across a radio link. It was either buy a Canon flash or forget it.
Early on, and in most inexpensive wireless flash controllers today, all the wireless connector does is tell the flash to fire. The flash fires at maximum, unless you manually scale the power back.
Even today professional photographers don’t use anything more complex than units like this Cowboy Wireless Flash Controller because they still prefer to use a light meter and set the flash power manually. Shooting that way is perfectly okay.
Some of the newer units like these can give you control over the sync and flash power (not full TTL support) over the wireless link.
But along came a photographer who was annoyed that his e-TTL would not work across a radio link and, even though many others had failed, he partnered up with an engineer and figured out how to get it working. They called it The Radio Popper.
Other companies have since figured out the wireless TTL magic and now wireless TTL is available for almost any combination of camera and flash, if you want to spend the money.
Some people, even those who have the money, still prefer to use a light meter.
A wireless data link is not your only option. If line-of-sight is not an issue, you can still go with an infrared transmitter. There really just isn’t much incentive to buy infrared over radio these days. It’s so convenient to be able to put a flash unit out of sight in the background that limiting yourself to line-of-sight seems silly.