They still work when blunted which is most of the time. But ideally they are sharp and slicing pure grass, grass tolerates slicing better than ripping. It's like a sword, ideally it is razor sharp, doesn't chip when blunted, and still works as a decent club even when blunted because it will be the second it hits something hard.

For regular mowing sure. For reel mowing (e.g. golf) you will want to sharpen them.

I have a battery electric lawnmower. I found I got noticably better battery life after sharpening the blade to a near razor edge. Yes the edge is more fragile but it cuts the grass with much less resistance.

That I did not know. I still wouldn't stick a finger near one of course.