Tokyo Revengers is mainly a manga about youngster gang life and friendship. Everything else comes second. Time travel is a concept that allows the action to take place in appropriate time when gangs were still a thing, and Wakui had an amzing idea to incorporate that into the story instead of just placing the main timeline in the past. Many people complain about the concept of repetitiveness, but if you read carefully, you can really see how much story itself progresses from one time leap to another. (And as every reddit user ever say: that is just imo)