Here are seven reasons standing will improve your climbing ability.
1. By adding your body weight into the pedal stroke to increase your power output. With proper technique you can efficiently add your body weight into the downstroke to create more power.
2. By shifting your weight forward on steep terrain to gain balance. Moving your weight forward over the bike by standing rebalances you and your center of gravity giving you more stability and control.
3. By adding leverage to create more power from your effort. This tip is an extension of #2 as getting more forward really allows you to leverage your weight and achieve a better angle over the cranks into the pedal stroke.
4. To create a faster acceleration. This tip builds off of #3 as these details allow a higher and faster power transfer into the pedal stroke getting you up to speed faster. One thing to add here is that getting up to speed faster becomes more efficient as it takes less time using more energy to create speed.
5. To increase your speed up and over the tops of the climbs. This is an extension of tip #4 but try to look at every crest as a place to finish strongly and get as much speed as you can.
6. To control your effort on steeper gradients. By standing in a lower cadence you are able to lower the power you are producing by adding your weight into it.
7. To raise your speed out of corners or switchbacks. Counter intuitive, but the switchbacks are really where you need to look to build speed, not recover. This speed will make the rest of the climb easier and faster.