Standing vs. seated climbing

Seated climbing is more efficient because you do not have to support your body’s weight. But standing climbing - on road or off - can be a more racy option. Think of it like running because you support your entire weight on your legs and with training runners can go for tens of kilometres in a race.
The key statement there for the runners is with training.
Similarly, to do this well on a bike requires specific training. Most of which happens off the bike.

Skipping.
Lunges.
Squats and deadlifts (especially one-legged, like pedalling).
Kettlebell swings.
Anything core strengthening.

These work on leg strength, balance and stability. Essential ingredients for comfortable and fast standing climbing.

Recently I must have passed some sort of threshold with regard to becoming efficient at standing. I was able to clean a new line on a multi-choice technical climb on the mountain bike and I did some decent pitches standing on the road bike without thinking about sitting.

I can still sit and spin, but now I can stand too. Versatility is always good.