The way your fingers learn to play an instrument kinda goes like this:
There are various parts of your brain carrying the instruction to and from your fingers (remember, your fingers also tell your brain what state they're in, what they're touching, etc.): it's not just the muscles that have to contract, it's the knowledge of what sound it will make when they do, the meta-knowledge around that sound (where it is in whatever key you're playing, its role in the melody or chord, what emphasis you're giving the note, etc.), memories from when you learned and practiced playing that note, and so on.
The first time you play, a signal streaks through your brain like a lightning bolt. Just like a lightning bolt, it twists and branches as it touches each area of your brain mentioned above. (They've actually mapped this, it's fascinating.) When you practice and play that note again, that signal repeats the same route. This is complicated, and so it feels difficult. (That's why it's easy to wiggle your finger open and closed--that's just pure muscle movement. There's not all that other stuff to think about.) (That's also why it's hard to correct a bad habit.)
But just like a trail being blazed through the woods, the more time that signal travels that path, the faster and easier it becomes. You find you don't have to "think" as much. (And my argument earlier was that you don't have to "feel" as much either.)
Well, that same process applies to creating music, even without an instrument in your hands. In that case, the difference is that instead of ultimately controlling muscles in your fingers, you're controlling your ability to come up with a new melody, or chord progression, or a twist on an existing melody, or rhythm, etc. It seems weirder because the "muscle" being trained is inside the same brain that's controlling it. But it works exactly the same way.
I've been talking in sort of neurological terms. When I said "plus one thousand" to a previous comment about how we're all different, this is what I was referring to. The brain is an organ. It's a fatty mass of cells sitting in your skull. Just like the cells that make up your leg muscles or your skin, their characteristics are largely determined by your genes. Humans share 99.99% of the same genes, so our brains are 99.99% the same, but it's a fallacy to think this means there will only be 0.01% variation in outcomes, because that 0.01% gets amplified by all the things our brains allow us to do.
This is why so many scientists tore Malcolm Gladwell's "10K hours" theory to shreds. (Just one example:
http://www.fastcodesign.com/3027564/asides/scientists-debunk-the-myth-that-10000-hours-of-practice-makes-you-an-expert.) I could practice golf for 10,000 hours and be a competent golfer--probably better than most people who play golf--but I will never be as good as Arnold Palmer or Tiger Woods because they have a genetic advantage. (And they're not autistic savants, either!) That 0.01% difference gets compounded at every twist and turn that a signal makes through their brains, just like a compound interest rate, so by the time we're both standing there with our golf clubs, there's a night and day difference explainable purely by genetics.
If we were all the same, then outcomes would just be a matter of who practices the most. But obviously, they aren't. Do black people just practice running more and that's why they keep dominating track and field events at the Olympics? No, they have a genetic advantage lying in the physical traits needed to run fast. Do Danes just practice standing up tall and that's why they have such a high average height? No, they have a genetic advantage lying in the physical traits needed to grow tall.
By that same token, if you believe in evolution, do you think humans split off from our ape ancestors by practicing walking upright? No, we had genes that allowed us to do that easier. Ours brains are physical objects no less subject to genetic influence than leg muscles or bone structure. Some of us will always be better than others at making music given equal amounts of practice.
That's why it's important to practice music as much as you can: you can't take a test to see how much musical talent you have, you'll only find out by making music! If you don't have much talent, you need more practice. If you have a lot of talent, practice will allow you to maximize it and help you use your talent to make big things happen in your life.
The only reason NOT to practice--to wait around instead for the right mood or something--is because you're not really serious and you don't really care about making music.
