This allows a user to override the automatically selected Device Pixel Ratio value. It is often either 1 or 2 by default, depending on your display's DPI setting.
An accurate DPI value is important for calculating the correct requested resolution of a video. For high density displays, you'll want to have a higher resolution of video, especially in the case of the Electron Capture app, where the reported resolution isn't the same as the displayed resolution.
Changing this value can provide for higher quality or lower quality video on playback, a bit like changing the &scale value, but dynamic to the current window-size of the video being played back.