I was referring to Starfish's climb of 5100 ft. in 19 miles! I just did the measely 2400 ft.

Standing uses up a lot of energy and is generally used just as you described, coming over the crest of a hill when you have just a short distance left. Also, it can be used on really, really long climbs, just to change things up a bit. You're more efficient when sitting. (I learned this all through other threads here, do a search for hill climbing and you'll find more answers to similar questions). As I become more comfortable with my bike handling, I also find myself standing to get through intersections more quickly when there's lots of traffic.