Attention is finite. To the degree that you can focus, you can conserve your attention in order to accomplish what matters.
In the image below, a flashlight is trained upon a page of sheet music. It's illuminating most of the page, dimly.
Now, we move the flashlight closer to the page:
As you would expect, the beam of light gets both brighter and smaller. The flashlight bulb puts out a consistent amount of light, so as it covers a smaller area it is more highly concentrated.
You can see where I'm going with this: your attention behaves the same way. The more focused you are, the less attention you waste, making you more effective. It's called concentration, get it?
This may seem like a no-brainer, but many players do not behave in a way that reflects an understanding of this concept. They start at the beginning of a piece of music and hack through it until it's over or until they run out of steam, whichever comes first. And then - horrors! - they might just go back to the top and try again.
A better approach: focus all of your precious attention on a tiny section and polish it until it shines. If you're doing it right, you'll be totally exhausted well before you learn the whole page. That's okay - there's always tomorrow.
Attention: A limited resource that you can make the most of by improving your ability to focus.
Can you think of any applications of this concept beyond learning a piece of music?