> I don't even know what hardware accelerated video playback in a browser is
It means that your browser uses the dedicated video card for rendering video frames and pushing it to your monitor instead of using the CPU.
Why you would want this? The same reason you use a straight-edge to draw a line and a compass to draw a circle. The right tool for the job. I'm sure most people here would have had the experience of the first time training their machine learning model on a GPU instead of the CPU and having their jaw drop at how much faster it was? GPUs are simply much better than CPUs at the things they are designed for. Sure, most CPUs are still powerful enough to push 1080p videos to the screen, but that comes at the cost of poor performance and battery life.