Because the course does not start automatically

Well... that is part of it. 😉 Another part is a site (an LMS, for example) which opens a new tab for a course, but doesn't give it focus. If a browser tab does not have focus, the embedded media content is prevented from automatically playing UNTIL the user "gives" it permission to play (by clicking on the PLAY button). Another example would be you would be reading a long article which includes several hyperlinks which you right-click, open in new tab on, only to have the embedded video start playing (if you right-click a hyperlink and select "Open in new tab," the browser maintains focus on your current window. It does not give it to the new window).

Even legitimate sites sometimes intentionally opened additional child windows behind the current parent window using the window.open(); Javascript method. AND, using Flash, have annoying advertising play, which would send you diving for the volume control. I, for one, like that they are stymied from doing that.