The core idea is that there's a lot of useful context about the people you email, but email clients right now don't give you any of that context. We serve it up right next to your email, so you don't have to hunt around for it. We want to help you engage more meaningfully with the people you email.
We've found different people find it useful for different reasons. Founders, journalists and BD people get a lot of mail from people they don't know, and we've had a lot of love from that demographic because we help them put a face to the email. But as you say, a lot of people get mail mostly from people they already know.
We've got some ideas coming up for that which we think are going to be incredibly useful. Right now, if you get an email from a friend, you can see their recent tweets right there in your email - I've found this really useful, as I can refer to their tweets while replying to their email, or see who else they've been talking to. There's a lot more coming along those lines.
Is there anything we could do that would make it indispensable to you?