This image is know as test card (or test pattern) and was originally used as a TV test signal. Before 24/7 TV, it was typically shown when the regular broadcast ended. In its beginning it used to be physical test cards at which the camera was pointed and was later replaced by a digital version.
See Wikipedia for more context.
In Youtube'sYouTube's case, I can't really see any value other than an homage to the earlier days of video broadcasting.