Think of it this way. Digital cameras are generally made by companies that do other consumer electronics. So, the first wireless controls for off-camera flash that were built were based on existing IR* remote technology—like how your TV remotes work. TransmittersThis has an obvious advantage for manufacturers of not requiring radio bandwidth allocation in all the countries where they want to sell their product. Optical/IR transmitters and receivers communicate by a series of light pulses, and how those pulses are interpreted are typically by a brand-specific protocol. Third party flash makers, like Nissin, et. al. reverse-engineer this protocol to make flashes that are compatible within those systems.
Like a TV remote, you have to point your transmitter at your receiver or bounce the signal off of surfaces for things to work. Using the pop-up as your master and placing your flash behind you, or trying to hide the flash behind a door, or use it outside a window to fake sunlight--these are all relatively problematic with an optical system, but easily accomplished with a radio system. And TTL-capable radio triggers do allow for remote power control, which becomes extremely handy if your flash is someplace inaccessible or you don't want to have to keep ripping open your Westcott Apollo softbox to get to the flash to adjust it. But you do have to worry about RF interference, since to get around bandwidth allocation issues, most of the newer radio triggers use the 2.4 GHz worldwide ISM band, which is crowded with lots of things, like wi-fi and Bluetooth.
