Autohide taskbar means you also get some extra screen space back for fitting in more application elements, useful in apps like Photoshop or a vertically pinned browser window. So it's a useful feature. As for background, I have a folder of photos I have taken over the years which rotates every 10 mins so that's all sorted for me.
Not by default because the Nvidia driver sets the scaling to the display, and the DW does not have a standard scaler so it essentially stretches the 2560x1440 image to fill the screen, any scaling to aspect ratio etc has to be done on the GPU. You can fix this by going into NVCP and setting it like this:
So if you do have to use say 2560x1440, or 1920x1080 etc, then you will get pillarboxes as is expected like this: