The Justice League Dark deals with a lot of supernatural threats, be it demons trying to end the world, or the time an Old God tried to destroy magic. This makes their job just as important, if not more so than the regular Justice League, which makes it all the more imperative they can get to where they need to be on time. Finding supernatural threats isn't as easy for Justice League Dark as finding where Darkseid is invading or if the Legion of Doom is trying to take over the world again is for the normal Justice League, but thankfully, John Constantine has a skill for that.
John is a man of many talents, such as magic and manipulation, but the one ability he has that would undeniably aid his team the most is Synchronicity Wave Traveling. This ability is best described by Boston Brand, "He's riding the synchronicity freeway, and so everything just falls into place; time, movement, even distance just sit up and beg for him." While using this ability, John can always be where he needs to be, regardless of time or place. This ability isn't to be confused with teleportation though, it's much closer to luck. Though the Justice League occasionally disrespects Constantine, this ability is incredibly useful. When the mage hunter Mako was killing mages in Africa, one of the mages fled to England, having to take a journey of weeks, going from ship to ship to travel from Africa to England, and when he finally reached England, he found John in just a few seconds after stepping off the boat. John had simply decided to go on a walk that morning and happened to walk by the harbor, with no prior knowledge of what was coming. But he needed to be there to see the mage when he arrived and so he was.
This ability would be unbelievably useful if he could use it for his team, being able to simply be where the supernatural was manifesting would allow Justice League Dark to stop monsters before they managed to cause any real damage. When the Upside-Down Man was terrorizing the world and Justice League Dark, he was always one step ahead of the team. But with John's ability, he could've been exactly where he needed to be, exactly when he needed to be to help foil the Upside-Down Man's plans. When Justice League Dark was facing off against the mad wizard Merlin, Merlin was constantly one step ahead with JLD constantly playing catch up to his plans, but John was there the entire time. With his ability, he could've been where Merlin was going before he was there, which really would've taken a lot of work off of his teammate's plates.
John Constantine is an incredibly powerful mage and it's hard to blame any writer who wants to focus more on his magical side or his conman side, both are equally important aspects of his character, but his ability to always be where he needs to be shouldn't be underestimated, and one day it may prove to be the ultimate upgrade for the Justice League Dark.