I have observed this many times for long haul flights where they have extra cockpit crew: during takeoff they all gather in the cockpit and then disperse to their rest areas. They meet again somewhere halfway to hand off flying responsibilities and then gather again before landing. Usually approximately half an hour after/before takeoff/landing.
One way of thinking is, if you are going to rest, rest from the beginning. Just take over your duties when your turn comes up. Why even bother visiting cockpit if you are going to be there only for first 30 minutes or so of the flight.
I understand takeoff/landing are most critical phases of the flight where things can go wrong. But then airline have deemed that whoever is flying the plane at that point is fully capable of flying or in other words fully accredited crew is managing the flight.
Is there any other reason why all the crew has to meet in the cockpit during these phases?