I've found that I am much more efficient when seated, and only stand for the really ridiculous climbs of say 15% plus like the one pictured below which hit about 19%

I see a lot of guys standing and I've tried it but I feel like it is easier, but I am not able to move as quickly.

Can someone explain it all to me? thanks!

Oh, and I fell in love with this climb. 300ft up in 1mi really had three climbs. First was about 11% second one was 9% then the last one was 19% by far the hardest climb I've ever done.