Adam is an incredibly important man, but not for what he actually does in the poem. He's important because of what he's destined to do. You see, he's God's first-born human – the "First man, of men innumerable ordained" (8.297) – and so he's the root from which a magnificent family tree (if we do say so ourselves) known as the human race will develop. In Milton's words: "Out of one man a race/ Of men innumerable" (7.155-156).
Now we know Adam isn't the most exciting character. He's kind of dull and he puts too much stock in Eve's beauty. Yeah, she's really, really, REALLY beautiful and hot, but his love for her is partly the cause of his ruin. Near the end of Book 8, he and Raphael are discussing this very issue, and Adam says all kinds of stuff about how astonishingly gorgeous Eve is. Adam gets so carried away that Raphael tells him to chill out and not let her beauty cause him to make bad decisions. It's almost like Raphael has to tell him not to think with his you-know-what. Now, this becomes really important later because Adam ends up doing what his wife encourages him to do (dividing their labor; eating the Forbidden fruit) because he can't refuse her beauty.
The fact of the matter is, we all see aspects of ourselves in Adam. You know how hard it can be to resist the pleadings of your super-hot boyfriend/girlfriend? Surely we can be just a little sympathetic. And Milton is somewhat unclear as well; after the Fall, he says that Adam was not "deceived," but rather "fondly overcome with female charm" (9.998-999). Is Milton saying that Adam wasn't tricked because he's too smart for that? Or is he saying that Adam is so weak that he fell prey to his wife's "charm[s]"?
Either way, Adam (and Eve) pays the price; he has to leave Paradise, and on top of that he has to learn about the horrible events (narrated in Books 11-12) that will happen as a result of his and Eve's behavior. In some respects, then, he resembles other literary characters entrusted with the burden of painful knowledge (Jonas from The Giver comes to mind; head over to our guide for more!). Notice how Adam doesn't tell Eve about all this stuff; we don't know if he ever does. In some respects he's like a parent or older brother that knows lots and lots of bad things that he doesn't necessarily want to share with his younger, more innocent siblings. Just think of him as someone who has to watch horrible war footage that the public isn't allowed to see. Would you want him to tell you about it?
But not all knowledge in this poem is bad, however, and Adam is also the figure through whom Milton reveals his own theories about what God was really up to with that whole Tree of Knowledge business. People have often complained that, because the tree was forbidden, knowledge was forbidden. The Bible doesn't really say anything about this, so Milton fills in the blanks with his own theory. In Paradise Lost he argues that Adam and Eve will eventually acquire the knowledge they don't yet possess. This is the whole point of Raphael and Adam's conversation in Books 5-8. In Book 5 Raphael tells him flat out that eventually, he will know everything he wants to know and then some. The real sin that Adam ends up committing, then, is not the knowledge he gains, but rather the proleptic (i.e., before the proper time) knowledge that he gains. In other words, he tries to walk before he's learned to crawl. Or, better, he takes his daddy's truck out before he's even gotten his learner's permit. Why do you think Adam's premature education is such a bad thing? What role does time play here?