Alt/Option is used for a lot of keyboard shortcuts, and in many apps, holding it down will alter the behavior of a button or something in the UI, cf. iTunes. (If anything, it would be better to ask about Control, but that's still needed for some Unixy and now Windows apps...not to mention that's an additional modifier for keyboard shorcuts.)
And brackets are used because, in addition to being standard on US keyboards, square brackets are often required in typing, and curly braces...well, besides math (which they are pretty much useless for at that size anyway), you need them for programming in many languages?I do wonder who decided those would be a good idea...
I understand why they're needed...but the alt/option key...2 names for one key is annoying. But I completely understand. I'm just a curmudgeon.