this is not quite what it makes out to be. of course at one level you only need an ethernet cord, but at some level you need to supply power to the cord itself. That power requires a power cable + PoE box if done close to the monitor, or you can buy a rack PoE unit and do it further away, which will have a limit on how far away from the unit you can be.

PoE units are useful when having only one cable enter the housing for waterproofing issues. otherwise, why go to all the hassle?