Self-Advocacy is important for EVERYONE.

From the trivial to the life-changing, the ability to advocate for what you want and need is important. Think about it. Let's say that you were taught that whatever went wrong or was not what you wanted, you could not do anything about it because you didn't really know what was best for you and that what you wanted did not matter. Let's say you were taught this through the actions of someone who thought they were doing what was best for you by protecting you from everything, but whose belief that you could never have autonomy over your life or live on your own showed in everything they did. And slowly, you came to believe them. It's a self-fulfilling prophecy, isn't it?
But if you were taught that what you want matters, and that if you bring up an issue, someone will listen and try to fix it, you learn that it is well worth the effort to try. You learn that you can tell people what you need, and that if they don't listen, that it's their problem, not yours. You learn to advocate for yourself. Sometimes it is frustrating- perhaps people only want to listen to your parents. But if all your parents will do is demand that you be listened to, occasionally, as a last resort, jump in with those who simply will not listen to a child, and sign the paperwork to make whatever it is you asked for legally consented to, you learn that you are in control of your own life.
Clearly, the thing to teach is self-advocacy if long-term independence (or long term interdependence on people of our choosing) is to be achieved.
Now that I've done all the nice rhetoric, perhaps you want to know how to actually do this. It's not that hard, really. (This is advice directed at parents, now.)

• Get some form of communication up. It's OK if all you can come up with is a way for two-choice questions to be answered, though it's certainly better if you can come up with a way for your child to come up with what they want and say it themselves.
• If it doesn't really matter (like wanting to wear a costume that doesn't violate the dress code to school, perhaps,) just go with it. It teaches your child that their wants do matter.
• If there is a good reason that they can't have what they want, explain why. The explanation can come later, if need be, but make sure they understand.
• Get input on the important things. REAL input. And take it into account.
• Bring your kid to IEP meetings and insist the teachers listen to what your kid says (or let your kid write something to them if your kid doesn't want to go/doesn't want to talk to them.)
• When something goes wrong for your child that you could handle yourself, but which your child also has the language skills to make an attempt at, offer them the chance to try.
• If they ask you to handle it this time, that's self-advocacy too- they asked you to do something.
• Remember that cruddy ice cream example on the Autism $peaks transition kit? Go ahead and do that if it comes up, starting as soon as your kid has the language skills to do it. Because for someone just learning to talk/just learning to use AAC/just learning to communicate in public methods, a fairly low-stakes self-advocacy isn't a bad way to start.

And yes, people who need help with daily living can advocate for themselves. I'm not entirely sure why this needs stating, considering that non-disabled children advocate for themselves on a fairly regular basis and needing help with daily living is considered part of being a kid, and also considering that a big part of self-advocacy is asking for that help and then getting it. If you can do it all yourself, you don't need to advocate to get other people to do things.

This has been in response to Emma Refuses To Get Off the Bus and A Self Advocate is Born! It's a great story, and it demonstrates how important self-advocacy can be from a young age by way of Emma noticing that the bus was taking her to the wrong school and refusing to get off in the wrong place. She did the right thing there. She said all the right things, telling them they were taking her to the wrong place, and in the end she simply refused to get off.