Playing position from the 9 ball to the 12 ball isn't easy. You still must play the shot hitting the rail first to play position on the 12 ball, and your window for opportunity is a small one. Yes it's possible, but difficult. Playing the shot in the fashion that Varner chose offered more chances to get things done, i'll explain why I believe that.I would have played the 9 to get position on the 12 but certainly not taking a chance to make or move the 15. The position on the 12 gives a chance to make another ball and break out the stack. With a good breakout a chance for a run out. Without a good breakout, then removing the 15 is much more controlled.
With the way he shot the shot he could have been thinking possibly two different ways. One way would be to pocket the 15 ball and take his chances on coming up with another shot, possibly on the 12 ball or the 10 ball. The other way he could of been playing the shot was to swing the cue ball deeply out of the corner, and then go toward the stack with a little speed to play position on the 12 ball by striking the stack before the 12 ball and play position on the 12 ball, and at the same time loosen up a few ball while doing so.
I really can't say for sure if that was what he was thinking, but I can say that after looking at the shot the way he played it those two options were possible, and available.
If the 15 ball wasn't in the picture, I would play the second option, by swinging the cue ball deeply out of the corner and then going into the stack with a good chance of coming way with a shot on the 12 ball. Using this method will also create a better chance of running out by opening more balls up.