What is my goal in informational interviews/coffee chats?

First it's always a good thing to have spoken to a lot of bankers when you're actually in the recruitment process. You'll be less nervous, won't say anything stupid and just be able to talk like a banker if that makes sense. I have found that it really depends on the people you speak with. Some will offer help without you asking : "reach out to me when you're applying" or "let me connect you to X at bank Y who'll be able to help" etc. Others you'll have to ask for help more explicitly

You've spoken to these people on the phone before, right? If so, then meeting them for coffee is the next step. The goal is to show them a continued interest in investment banking. Hopefully you will get another contact from meeting them, but the real goal is to give them more of a reason to pass your resume along during recruiting.

It's really a psychological move. When someone has met you face-to-face, it's more difficult to turn them away for a request. Also, you have forced the banker to spend more of their time on you. People are more likely to spend more time on people that they have spent time on in the past as opposed to just cutting their losses and running. Come recruiting time, the banker will at least subconsciously think "Well, this kid seems really interested, and I've already spent so much time on him that I might as well pass his resume along."

Make sure to dress the part. I would wear a full suit and tie.

Just make sure to ask a few questions that show you've done your research and try to simply be as likeable as possible. Also, leverage your current networks as much as you can (undergrad alumni, fraternity, etc...). You'd be surprised once you couple LinkedIn with a good directory.

End goal is to get your resume passed along to HR.

I have a bit different approach than what has been mentioned here. Treat it like you're interviewing the person/bank (not literally but mentally). Go in trying to figure out if you would enjoy working with that person, in their group, at their bank, and act accordingly. If you go in with this mindset, the questions and conversation will flow more naturally than if you go in with the mindset of trying to convince that person to help you.

Be in control of the conversation but don't dominate the conversation, if that makes sense. Be the one asking all the questions that will make them a). Talk about themselves a lot so they feel good and more comfortable with you and then b). Ask you questions that you want them to ask you. My main goal was to always establish a personal connection (which was easier done when we talked about football or college rather than "the culture at your firm"). They'll be much more willing to send your resume to HR and more often than not, will be quite impressed by your proactiveness and your grit. Lots of good points I used were also covered in the comments above.

Think of the coffee chat as one interaction within a greater battle plan to get the position and an underlying philosophy that goes something like this: I did everything to achieve my objective. You never know when a particular convo will be the tipping point for a reference, that will lead to an interview, that will lead to a job. And, of course, speaking with someone in-person is a lot more effective communication-wise than talking over the phone, through emails, texts, etc.