That "necessity" point is an interesting one. If I always caught my child before she fell, perhaps she'd learn to depend on me to keep her from falling every time she walked. Perhaps she'd even started getting angry at me when I didn't get there in time. (More likely, given human nature, she'd start glaring at me whenever she made her walking attempts, saying "Me do it!") In other words, it's possible to artifically tamper with the connection between necessity and learning.... to break the circuit. It's also a strong temptation for an older, more experienced person to want to "help" the younger novice too much. You see that with the 5 year old officiously "helping" his little sister walk, the toddler usually bitterly angry and both of them in danger of falling : )
It seems to me that our teaching or non-teaching methods have "meta-lessons" built in. I see the "meta-lesson" in unschooling (ideally) as being confidence in the human ability to learn new things for good reasons. An unschooled child might perhaps be more likely to jump in and learn to do his own taxes, as an adult, than a conventionally schooled child who was taught by implication that he had to be taught everything new.
At any rate, most of the adults I know who are willing to learn new skills through life are people who as kids developed confidence in their own ability to learn. And often, from what I've seen, this confidence came from the "unschooling" (self-directed, independent) part of their life. In other words, what they did because they chose to, freely.