instincts
You gotta have a fairly straight stroke first. This normally requires hitting 1000's of balls before you have some idea about where the cue ball is going.
But the eyes and where they are set over the cue is more strongly related to hitting a chosen target. Therefore, if you have hit 1000's of balls and have some idea of where the cue ball is going then you should be a believer in focus (or aim).
In other words, I don't care how straight a stroke, set-up, pre-shot routine or anything else that you have developed, if your eyes are not coordinated and focused on the target then good luck.
I've seen the best strokers of the cue and I have seen them miss. IMO this means they are not fully connected to the target, thus aim becomes more important.