If you don’t use it at your office, you might have never heard of Slack. But plenty of tech startups and traditional companies say they can’t function without it. With 5 million daily users and rising, this barely three-year-old app is quickly becoming an essential tool in all sorts of businesses.

Each technological advance seems to follow an established adoption lifecycle: a few innovators, a few more early adopters, and then a steep increase in users when the majority being to accept and use the product. Geoffrey Moore described this cycle in the book Crossing the Chasm, where he writes “in high tech, the good news is that, although we lose our companies with alarming frequency, we keep the people along with the ideas, and so the industry as a whole goes forward vibrantly.” Slack was created by Stewart Butterfield, who previously co-founded Flickr, and who created Slack somewhat by accident as an internal communication tool for a gaming startup. Slack has some hints of those other businesses, with a friendly interface and focus on interactions.

Yet the premise wasn’t as innovative as a game or a photo-sharing app; instead, it was simply presenting an alternative to email, the backbone (and scourge) of every office. Despite many attempts to improve the antiquated platform with sorting and folders, it is essentially the same concept year after year. Some chatting products fill in the gaps, but they’re only best between a few people. While intranets and wikis or group forums allow for longer-term storage of discussions, they quickly become cluttered.

While Slack started out small, it now counts among its customers NASA’s Jet Propulsion Laboratory, IBM, Ticketmaster, LA Times, and The New York Times, which was so excited about Slack it printed an article titled “Slack, the Office Messaging App That May Finally Sink Email.” Finally! But what makes it so appealing? Slack is free up to 10,000 messages, and then either the history gets lost or the paid version is available for a subscription fee.

While these features are excellent, no one of them is truly innovative by itself. Here are four reasons Slack is different, exceptional, and here to stay:

Slack is hosted online
This is simple but essential. There are still plenty of email products that pull information to a local device such as your laptop–which means it’s no longer accessible from another device such as your phone. Slack is hosted online, so it stays in sync across desktop and mobile devices for people who are on the move. That seems essential for a communication tool, but it’s not nearly as ubiquitous as you might think (for example, a text message is only hosted on your phone, and not on the cloud). This also makes it much easier for company tech departments to implement Slack.

Slack is customizable
While it takes a little getting used to, Slack has several levels of customization and many features that make it a great way to have short chats, send files, engage groups, produce updates…the options are endless. Slack itself outlines some best practices, such as how they use Slack for tracking thousands of job applicants and onboarding new hires.

Some of the functions that make Slack so adaptable are:

-Teams: A Team is a group, and it can be set so members have to be invited or otherwise approved, or it may be open to all.
-Channels: Channels on Slack are given a #, and can be secret or open. Channels could be used for example to convey all company messages or smaller team updates and can be created flexibly to allow for short-term teams or other agile business practices. Some businesses have select channels open to clients so they can see what is happening as well.
Direct chats or group chats: Like traditional chats, messages are a great way to get someone’s attention briefly without going to find them in the office or creating an email. They’re also a good way to share links, documents, notes, or even just to let people know messages such as when someone is running late.
-Integrations: Add Google Docs, connect to GitHub, link to Zoom, Zendesk, Salesforce…the third party app integrations are endless. For example, a conversation about a client can be directly tied to the contact in Salesforce without leaving Slack. Once the app has been connected, someone might use the third party app without ever going to log into it directly again. That’s powerful.
-Notifications and muting: Slack can be configured to notify you whenever a keyword occurs in a chat or there is another kind of activity that you want to monitor. Channels can also be muted when you need to get work done without being disturbed.

Slack has a searchable history
Anyone who has left the office for a few days dreads coming back to a full inbox, but equally harmful is the loss of information from smaller conversations and updates. Slack is searchable, so someone returning from vacation could either scroll backward or search for keywords to catch up on anything they’ve missed. Creating transparency is part of the company mission to “turn institutional knowledge into common knowledge.” Through machine learning, the search even adjusts to give more relevant results each time you use it.

Slack fosters human interactions
Slack’s secret sauce is the ability to have a watercooler conversation with anyone, anywhere. Colleagues can start up a channel for #celebrations or #birthdays and congratulate one another on big wins from anywhere in the world. These kinds of notes could be passed before by email or messenger, but within Slack, they are doubly enforced by the group. The addition of emojis, animated .gifs, or photos all makes Slack feel more like a friendly conversation space.

Slack is so simple and fun that even social groups and other informal groups use it. Clubs and meetups might use it to organize events, while professional circles form around topics and gain members by word of mouth. Some groups charge monthly fees for members, while many are free and powered by enthusiasts.

In early 2017 Slack reported to Forbes a total of 5 million users and a valuation of $3.8 billion. Machine learning will only amplify its most positive features. Slack already is beginning to integrate personal-assistant-type functions, with a bot that can remind people of their daily calendar or follow-ups. As Slack expands internal features, the third-party integrations and creative community groups are advancing it at an equal or faster pace.

There’s no slacking in the timeline of Slack’s success.