Didn't you just answer your own question? If a biological feature makes an organism more capable of survival, then it will be selected for.
Really? So, monkeys don't use their thumbs? Pandas don't use their thumbs? Dogs don't use their dewclaws (which are basically very simple thumbs)? Without intelligence, the thumb would still be a very useful appendage. You might as well say that if we weren't intelligent, we wouldn't know what to use our lungs for.
Why, SackLunch, what ever made you think I'm an "evolutionist?" The "ism" suffix implies a fixed attachment to a particular belief. As a scientist, I would never attach myself to a particular belief about science for fear of blinding myself to other possibilities. Of course, since the Theory of Evolution is so abundantly supported by evidence, and since no other theory of biological origins can claim the same, I don't doubt its truth; but I'm no "evolutionist" any more than I'm a "gravityist."
As for your question, you already answered it above. The thumb increases our survival advantage, and so the mutations that created it were selected for.
Well, that's an easy one! The thumb didn't come about by random chance. Wow, you should really file a complaint against your biology teacher -- I can't imagine how you came to have such a misconception. It's too bad when our education system fails like that, wouldn't you say, SackLunch?
But to correct your misconception, random chance is only a factor in mutation. Mutations, which are random, provide the raw material for evolution. Then the process of natural selection selects from this random material to produce entirely non-random biological features. Simple to understand once it's explained, isn't it?