I've watched some pro-games (LCS, All-Stars, etc.) and really high-Elo games lately and noticed that the towers are always shot down to about 5% health but it is left alone and not destroyed. (When you want to kill a champion below such a low tower you should destroy it)
Why is this done?
Should I be doing this in normal games and low-Elo ranked games?
Best Answer
This is done to deny your opponents farm. If you can push your opponent out of the lane and let their tower take the minion kills, then they are missing out on valuable experience and gold.
This also prevents your lane from pushing too far, which might put you at a disadvantage where you have to farm too far away from your own base.
The tower is pushed to a low amount of HP so that, when the need arises to take the tower down later in the game, it's really quick to finish it off and move onto the next objective.
In lower Elo, this isn't actually as important. Your opponents were probably going to miss some of the last hits anyway, and they will likely push your lane back harder allowing you a safer farming position, or telegraph their gank, or not even gank you at all if you're in an extended position. It doesn't hurt to try to play optimally, but you probably won't get noticeable gains from doing this compared to in higher Elo.