Warding the middle of mid lane means that if the enemy mid shoves the lane to go roam top or bot, the ward will spot if he begins running toward top river, bot river, or back to his tower.
It doesn't show his final intended destination, but it does give a good overview of which direction he's heading in, so you can make a solid guess at whatever he intends to do.
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This is a very good ward against a mid laner like Twisted Fate. If you a ward in top river spots TF moving up from mid lane, your top laner's likely already in Destiny range. Vice versa for bot lane. A ward in the middle of the lane gives less definite, but earlier and more holistic information about where he might be headed after shoving lane.