Centuries before the events of Justice League, Zack Snyder imagined a DC Universe where Darkseid had come to Earth in search of...something.
A piece of concept art apparently depicting a young Darkseid caught Snyder's eye on Vero, where he commented that the mad god was "searching for something that would make him unbeatable," on a planet that appears to be Earth.
The most likely answer is that he was searching for the Anti-Life Equation, Darkseid's MacGuffin and motivation for most of the stories in which he appears.
During interviews conducted while he was still working on the Justice League films, Snyder had suggested that Darkseid would eventually come to Earth as part of his quest to find Anti-Life. It was that quest which was tied into the visions seen in the "Knightmare" sequence from Batman v Superman: Dawn of Justice, as best as fans can recall. A good deal of discussion about various interviews that Snyder has done can be found in the comments below the image, which is embedded below.
Originally, Justice League was meant to span two films, but that idea seemed to fizzle even before Zack Snyder left the project. The popular theory was that Darkseid would not appear until the second movie, picking up where his uncle and chief general Steppenwolf had failed.
Between the changes the film underwent after Snyder's departure, the jettisoning of a planned sequel/second half, and a cone of silence that has descended over everyone involved, it is difficult to guess exactly what form Darkseid's story would have taken, and how differently it will be handled when Ava DuVernay makes The New Gods at Warner Bros. in the years to come.
Snyder left Justice League mid-production. The studio was not happy with his cut, and it was clear that significant reshoots were going to be needed in order to bring the film into line with expectations. Meanwhile, his daughter passed away, and the filmmaker decided that it was best to step aside and allow Marvel's The Avengers director Joss Whedon to complete the film.
Upcoming DC Extended Universe films include Aquaman on December 21st, Shazamon April 5, 2019, and Wonder Woman 2 on November 1, 2019.