Wireless cameras are not completely wireless. They still require a power source, which means that they must be plugged in.
The wireless aspect takes place between the wireless receiver attached to the DVR and the camera itself. Wire-free security cameras, on the other hand, run on batteries or solar power, which means they do not need to be plugged in. These cameras typically only turn on if motion is detected.
Without a constant power supply, however, wire-free cameras will not provide 24/7 surveillance, will need to be constantly maintained to prevent battery issues, will have trouble in high traffic areas since the constant motion will quickly drain the batteries, and will lack the required power for long-range night vision.