With the Flash going on a timetravel adventure in The Flash, Barry Allen is gonna find himself in a world similar to the events of Man of Steel. That movie, that like The Flash, makes use of jumping around time to tell a story. Albeit The Flash does it through timetravel, and Man of Steel only through narrative timejumps in the form of an extenstive prologue and several flashbacks. Both do this to give the viewer additional information or context in service of the main narrative.
Showing these pieces of information or context, the viewer gets a visual picture of those events. In Man of Steel, the viewer sees a young Clark experiencing x-ray vision, and later Zod experiencing the same thing. Because the viewer saw young Clark experience it first, it is made clear what kind of obstacle it is for Zod and thus the struggle doesn't have to be shown for him. Characters could just speak of these events, but it is more effective to show them.
Flashbacks
Man of Steel uses a prologue to show events explaining Zod's exile, Zor-El's final act and Jor-El's importance to Zod. However, several flashbacks and information given to characters and the viewer later in the movie re-explain this information. Some flashbacks itself suffer from this as well, leading to the movie wasting time on repeating things in the present. Flashbacks thus prevent the movie from picking up pace. A problem Eternals has as well.
In Eternals, the flashbacks are used differently. They don't provide any more information that already could be deduced from the present day storyline or even provide contradictory information, disrupting the flow of the movie in the process. Showing Phastos denounce humanity in a flashback stand in contrast to the following scene where Phastos has a loving, human family. Love and Monsters does this better. The flashbacks show Joel's inabilitity to perform a task, and the present-day shows him overcoming that. It follows each other up without contradictions or slowing the pace of the movie.
Different point of views
Flashbacks can be used to retell an event from the perspective of another character as well. ZeroZeroZero, Living with Yourself and The Last of Us Part 2 all use this technique. It does give information that is already known to the audience, but as it is told from a different point of view the nature of the information changes. All of the sudden, the viewer is given more context that might lead to earlier information being reassed in a different light. As a result, the entire story changes.
Different point of views can be happening concurrently to each other, and possibly even affecting each other. In Lord of the Rings, the storylines are interlaced (1). While one focuses on Frodo and Sam trekking to Mt. Doom, the other focuses on the rest of the Fellowship doing everything to distract Sauron from detecting Frodo and Sam. And because those storylines are connected, it's easy to keep up. Condensing timelines, like Rings of Power does, can also make it easier to connect storylines.
Intersection
Timejumps and interlacing can be combined as well, like Westworld or House of the Dragon do. In Westworld, the viewer sees William's actions in the past timeframe affect his behaviour in the present timeframe as the Man in Black. In House of The Dragon, characters' actions and experiences inform or even mirror past events when time jumps ahead years at a time. In both, the viewer is informed of events at the right time and in the right context only.
In short, timejumps, like prologues and flashbacks, can give the viewer additional information that otherwise couldn't be given. By showing events in different times and places at the right time, possibly from different perspectives, a movie's flow isn't interrupted. As long as it isn't information that is not relevant or repeated later on. Unlike with Man of Steel, that functions just as well by starting the movie at Clark arriving in the Arctic.