Everyone on the trip edits the same itinerary.
Your friend adds the restaurant while you are adding the hotel. Both stick. No itinerary-final-v3, no one person stuck retyping the group chat into a spreadsheet, no wondering whose copy is the real one.
The whole trip is on your phone. You need the itinerary most in the twenty minutes after landing, which is exactly when you have no data and everyone is looking at you. Open the app on the plane and it is all there, including the parts somebody added while you were boarding.
Editing works offline too. Two people can plan on the same flight, on separate phones, with the seatbelt sign on, and nothing is lost when you land.
Put dinner at seven. Book the flight afterwards and dinner is still at seven — it just moves to seven in the city you will actually be standing in. Nobody has to think about offsets, and nothing quietly slides three hours because of a flight somebody booked later.
Type three letters of an airport and it fills in the rest, the city, and the time zone at both ends. Hotels, restaurants, and anywhere else come from a map search — or you can just type the name of the place your friend recommended, which the map has never heard of.