Play Button Size
There is also an option in the Global Config tab in the ACP that reads "Play Button Size" which has a default setting of 'Disabled'.
This is for use in conjunction with when the Expanded Media is NOT enabled.
So the link is put there in the post, but to distinguish it from other links which would normally take the user to a different URL, in this case a play button is shown before the link.
Clicking on either the link or the play button will reveal the player.