In Zack Snyder's Justice League, Steppenwolf utilizes Boom Tubes, an extremely efficient mode of transportation to scour Earth for the three Mother Boxes. Although it's clear he has an array of supernatural powers, such as the strength to kick around superheroes like Wonder Woman and Batman, the film does not explicitly outline how the portals that conveniently teleport Steppenwolf actually work. However, they are not new to DC, nor are they completely of Zack Snyder's own creation. Boom Tubes are essentially wormholes for interstellar travel and are a carryover from the DC Comics.
When Jack Kirby created the New Gods for DC, he instituted a means by which the nearly omnipotent denizens of New Genesis and Apokolips could not only swiftly transport themselves between their own worlds, but also to other realms within the DC Multiverse. Boom Tubes work similarly to wormholes in the sense that they carry passengers in an instant, without being constrained by space and time, and causing no damage to the user. Although Snyder's depiction of a Boom Tube shares a visual similarity to Star Wars tractor-beams, they have a direct connection in the film to Mother Boxes, a parallel that also exists in the comics.
In some of the DC comic books featuring New Gods, all the Mother Boxes, while boasting various other important attributes, act as the batteries, or power sources, for Boom Tubes. Consistent with the comics, Mother Boxes in Zack Snyder's Justice League act as the metaphorical key that unlocks a Boom Tube door. When Steppenwolf first comes to earth, invading Themyscira with a cohort of Parademons to take the Mother Box guarded by the Amazons, he materializes at the bottom of a Boom Tube. As a New God, it makes sense that Steppenwolf would use such transportation, but it is the Mother Boxes, not Steppenwolf himself, that activate his portals.
Wherever he appears via Boom Tube throughout Snyder's Justice League he is in the presence of a Mother Box. The vault at Themyscira houses one, the second is kept at Atlantis when he arrives, and the third, while carried around by Cyborg, not only calls to Steppenwolf's parademons, it also, along with the first two, can open Boom Tubes. Steppenwolf is not capable of harnessing the power of Boom Tubes without a Mother Box.
While in the DCEU, Mother Boxes have existed on Earth in dormancy for millennia, abandoned there ages ago after Darkseid was vanquished by Ares, there is a plausible reason for why they have not called to any of the New Gods before the events of Justice League. Superman's presence on earth is the vital safeguard. As a Kryptonian, the threat of his power is enough to ward against the Mother Boxes calling to Steppenwolf, or even Darkseid himself, but all that changes when Superman is killed at the end of Batman vs. Superman: Dawn of Justice. That is why the Mother Box in Themyscira calls to Steppenwolf, and opens the Boom Tube.
While Mother Boxes possess many other mysterious gifts, including turning Victor Stone into Cyborg, their ability to form Boom Tubes for Steppenwolf is a very important addition to the plot of Justice League. Without the Mother Boxes, Steppenwolf would have been confined to far less rapid means of transportation, making his quest to unite all three a much more difficult task and adding further context to the ordeal of his debt to Darkseid. Luckily for the Justice League, and Earth itself, a Mother Box also had the power to resurrect Superman, which offered the balance to them allowing Steppenwolf to travel freely around danger.