It makes sharing files between your PC and phone super easy. And if you set it up right, you can even browse your entire phone's filesystem from your PC as well. Great if you take a lot of photos or download a lot of memes on your phone and want to quickly offload them to your computer. It also lets you play and pause media from your phone, so if you're playing music or watching a movie on your PC and need to pause while AFK, then it comes in pretty handy. Not only that, but it also has a presentation remote and can turn your phone's touchscreen into a trackpad and keyboard; great for HTPCs.
Those are just the features I use, it has way more than that. imho I think it might have too many features lmao
EDIT: Also, how could I forget about the feature that I mainly use KDEC for, the battery indicator plugin. It lets you monitor your phone's battery from your computer.
I'm confused. Are you talking about power over ethernet or power-line ethernet?