mjhjmtu

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?

There'a a few reasons for this:

Safety:
During takeoff and landing, everyone should be buckled into a secure seat. They have these seats in the cockpit, and in large aircraft, they're actually very comfortable. Laying down in a crew bed would not be allowed during takeoff or landing (though it happens, off the record). Do note that these beds do indeed have belts that are similar to those on rolling hospital beds, but it's not intended that these be used during takeoff or landing. It would be very difficult to escape the small confined space of the crew bunks during an emergency.

Redundancy:
If something goes wrong with one, or both, of the pilots flying, you would really regret letting the other two qualified pilots go lay down in the back of the plane. Safety comes first in aviation, and the pilots are there to support each other.

Extra Help:
Extra crew sometimes helps the flying crew, whether that be by looking up information, radio frequencies, airport identifiers, and so forth.

Socializing:
Pilots are (usually) pretty friendly to each other. They all share a common interest, and while some of the older pilots may be a bit grizzled, they all got into aviation for their love of aircraft. There's typically some pretty lively conversations in the cockpit. As noted in the comments, this is while on the ground, pre/post taxi.