Freedom - the ability to pick up my phone and be able to walk away with it if I want to without having to disconnect/connect a cable constantly (which requires two hands).
Before I got my 8+ I said the same thing and shook my head in disbelief. Now that I have an 8+ and 2 wireless chargers (one for bedside and one for work) - it's really nice.

Is it necessary? No, plugging in charges faster and works just fine. But it is less work and if I'm in a hurry, I don't have to spend that extra 1-2 seconds plugging in a cable, just set it down and walk away.