The 787-9 MLG doors open at lift-off and close after gear retraction or 30 seconds (?) whichever is first. It's a way of reducing the time taken for the gear to retract in the event of EFTO, the thereby reducing the time spent in the 'first segment'.
The 787-10 has the same design. You can see this in the numerous videos of the -10's recent first flight. The doors open immediately and then close again after a few seconds (I think it's 30, not 60). As it's a first flight, the crew deliberately leave the gear down.
The 787-8 does not have this feature.
Some of the benefits of a composite structure include:
Lighter, so lower fuel burn;
Stronger for the same weight, so bigger windows possible;
Similarly, higher diff pressure possible;
No corrosion problems so more moisture possible (higher humidity)