Jason Citron and Stan Vishnevskiy were video game enthusiasts. Citron had already had experience in creating games, but people didn't really like them. He sought to create something with Vishnevskiy to improve social relationships between gamers. The big plus in Jason's games were chats. So, the duo decided to put their expertise to use.
An easy way to communicate in games didn't exist then. Voice chat difficulties plagued TeamSpeak and Skype. So, Citron and Vishnevskiy came up with Discord.
It wasn't very popular at first. But, when one DLC for Final Fantasy XIV came out, someone posted a Discord link on the FF XIV subreddit. Jason and Stan clicked it to join the voice-chat with the newcomers once they spotted it.
People liked them a lot. Some users returned to that subreddit to urge others to come meet the creators of Discord. The tool experienced a boom of new members at that time, and that was the moment when Discord user acquisition strategy started to work. Since that day, Discord user growth has been just a snowball effect.