The Flash has been traveling through time for almost as long as he’s been in comics and originally he did this with the aid of The Cosmic Treadmill, which allowed him to harness his super-speed to power a time-machine and travel to any time in the history of the DC Universe.
The Cosmic Treadmill debuted in The Flash #125 — just two issues after the landmark “Flash of Two Worlds” story which reintroduced Jay Garrick — in a story which saw The Flash and Kid Flash travel to the far future and distant past in order to save the present from a destructive trace of aliens known as the Dokris using the Cosmic Treadmill, which Barry Allen himself invented and built because that's the sort of thing that happened in the Silver Age.
The Cosmic Treadmill became standard over the years, though in more recent stories The Flash has been able to travel back and forth through time without it. Going back to the 1990s, Wally West often found himself stranded in an unfamiliar future due to running too fast, while the now-infamous Flashpoint saw Barry Allen race back in time in order to have his mother from the Reverse Flash, only to return to a dark and disturbing dystopia forged by his carelessness with time travel.
The Flash’s love of time travel has also become somewhat of a meme in regards to the television show, with the trope becoming so common that fans have started referring to Barry as engaging in a lewd act with the timestream as a reason why things are different or unpopular in the show.
Without access to time travel, The Flashes are forced to abandon a special ability which had admittedly become a crutch, and resign themselves to operating only in the present day. That’s going to be hard when Wally West knows that his children are out there somewhere in Hypertime and Commander Cold is desperate to get back to an era which he doesn’t know no longer exists.
There’s also the still present threat of Hunter Zolomon who is still at large and will only be more dangerous than ever now that he no longer has access to the timestream; Hunter hasn’t been forced to live linearly for some time, and it just may end up sending the already mad villain over the edge. Plus, Bart Allen is back in the present day, which is cause for celebration, but as a refugee from the 30th century, he may be somewhat perturbed to know that he can’t go visit his mother or friends in the Legion of Super-Heroes.
The solicitations for upcoming issues of The Flash post-"Flash War" suggest that Barry Allen is heading deeper into the Multiverse to search for answers, with a trip scheduled at the House of Heroes which sits at the center of all creation. Barry recently visited the House of Heroes in Dark Nights: Metal and knows that if anyone can fix the damage done to the Force Barrier, it might be the multiversal heroes of Operation: Justice Incarnate and The Flashes of Infinite Earths. Combined with the problems of the Still Force and the six other hidden forces present in Justice League at the moment, it’s a bad time to be The Flash and things are only going to get so much worse before they get better.