It included a level editor and scripting language, which allowed anyone to unleash their creativity and design their own text-mode games using the ZZT engine.
This was absolutely unique at the time.
As a company, Epic is still hugely successful. Most gamers in 2020 have probably heard of Fortnite and Unreal Engine. According to Wikipedia, Tim Sweeney still owns over 50% of the company.
ZZT explored a lot of ideas about how to build a game creation system, which influenced Unreal Engine, which itself was hugely influential throughout the game industry.
The standard paradigms for today's game engines and editors was clearly influenced by ZZT.