[Slartie]

That's where it came from. Before discord, gamers set up actual servers with Teamspeak or Mumble and handed the IPs and ports to their guildmates. The "Teamspeak server" was where you met for voice chatting, and since Discord just wanted to supplant Teamspeak with their own product, they kept the "server" part in the name of the thing that gamers were supposed to meet for voice chatting.

It is a pure marketing ploy. Internally they call the "server" thing a "guild" AFAIK, which is technically just as much BS as the "server" moniker, but for other reasons (not every community using Discord is a "guild" in the commonly accepted meaning of that word in the gaming world, which is a long-term organizational group doing stuff together in an MMORPG).