While apps for work don’t usually inspire much excitement, there’s something almost palpable about the buzz surrounding Slack, a fast-growing communication app for the office.

Slack corrals all kinds of work-related messages, files, and alerts—things that often hog space in e-mail inboxes, instant-message windows, and elsewhere—in searchable, archived chat rooms that can connect to outside services like Dropbox and Google Drive, and are accessible on mobile devices as well as desktop computers.

Slack emerged from the wreckage of Glitch, an online game that Stewart Butterfield, a cofounder of Flickr, built with his company Tiny Speck but shut down last year. While working on Glitch, the four-person Tiny Speck team was divided between San Francisco, New York, and Vancouver, so they cobbled together a new communications tool by slowly adding features—like the ability to archive and search messages—to a simple IRC-like instant-messaging app. They used the tool so much, Butterfield says, that they stopped using e-mail to communicate.

After shuttering Glitch, the company switched its focus to popularizing the new communication tool. They dubbed it Slack and released it in August 2013. Since then, Slack has grown swiftly: more than 300,000 people use it each day, and the company has more than 73,000 paid users. The company has also raised a lot of venture capital funding—about $163 million since the company switched its focus to Slack.

Butterfield sat down with MIT Technology Review’s senior editor, mobile, Rachel Metz in the startup’s San Francisco office to talk about Slack’s sudden popularity, and to explain why he tells his employees that it sucks.

Let’s start with the obvious question: why is Slack so popular?

There are two real huge benefits that you get. One is a big increase in transparency. You can just see what other people are doing. You don’t need to have the standup meeting in the morning; you don’t need to have the status report or anything like that. It’s also across functional teams, so the engineers can see what the designers are working on, and the technical operations team can see what customer support is dealing with, and stuff like that. You can just dip your finger in the stream of information from some of these other channels.

The second one is that all communication that goes on inside of a company is the digital manifestation of its institutional knowledge. In most systems, that knowledge is e-mail-based and is very fractured and fragmented. Like everyone has their own slice. When you make those things available to everyone, they’re not only available now, they’re available to people in the future. So someone who joins the team next week or next month or next year, they don’t just have an empty inbox. That is, like every decision that got made, every discussion that ever happened, every time someone mentioned a source or a company or an institution, or anytime anyone shared a link or a document was exchanged, all of that stuff is searchable, you can just scroll back through it. It’s hugely valuable.

What new things are you working on?

When you use Slack there’s an insane flow of information, and that’s often overwhelming—it’s in real time, it’s distracting, and there’s just too much to keep up with. And that’s the flip side of having it all available. I think it takes time for organizations and teams to adjust to how to use it.

One of the things we’ve worked on internally is sentiment analysis. Imagine you’re the manager and you have an all-hands meeting. And you can see a live feed of positive versus negative language across the team, normalized to whatever the baseline is.

Are you bringing other changes to Slack?

Oh, God, yeah. I try to instill this into the rest of the team but certainly I feel that what we have right now is just a giant piece of shit. Like, it’s just terrible and we should be humiliated that we offer this to the public. Not everyone finds that motivational, though.

But there’s always room for improvement. There’s a huge amount of stuff we’re working on, from things like message replies and the ability to react to messages in a nonverbal way—one of the things people really like about Twitter, as an example, is to be able to “favorite” things. We’re probably about halfway through the list of things we wanted to do when we started two years ago. We still have a long way to go.