you need to wear a suit

Yes, you have to dress up for the interview.

I don't care if the office where you're interviewing is business casual. You can wear business casual when you're working there, after you impress them in the interview where you wore a suit.

I don't care if you don't feel suits reflect the "real you." I would wear head-to-toe fleece to the office if I could get away with it. But I can't.

I don't care if you don't like arbitrary rules like this. I'm not thrilled with them either, but I don't make those rules. I just want to know that you know what those rules are.

You need to slap on the suit and look professional. It signals that you take the job seriously. Sure, you might get hired if you wear a sweater and pants instead; I've hired people who wore that to the interview. But why wouldn't you want to play it safe and wear the suit? It's like thank-you notes -- if you're the right candidate, I'm going to hire you even if you didn't send a thank-you note after the interview. But if it's a close call, why wouldn't you want to do everything possible to give yourself an edge?

Just wear the suit.

(Disclaimer: This doesn't apply in certain fields, often tech-related ones. If you won't wear the suit, maybe that's the field for you.)