It gives you connections with your fellow grads throughout your life. You will enjoy the Shipmate magazine more than you can believe as a way to catch up on your classmates, folks from other classes, etc. You get access to the Mid Store for the rest of your life. There are other benefits -- check out the website. And, I'm sure that as we move into a more "electronic world," the benefits will change to reflect that.
I know when you're 17 or 18, your "entire life" doesn't seem all that long. When you're pushing 50 or 70 or 80, you have a different perspective. And you can't begin to imagine how close you will become to your classmates and how much you'll want to have an easy connection to them for years to come.
From a financial perspective, you'll never notice the amount that is deducted from your account given the thousands you'll spend on uniforms, books, A/N bathrobe bets, football tix, and the myriad of other things you'll pay for during your time. It will never be cheaper than it is now and, it's kind of like finishing your basement . . . if you don't do it when you move in, you probably never will.
Let me put it another way. You're committing to USNA presumably b/c it is your strong desire to attend the institution. As I said before, why wouldn't you want to join its Alumni Association?