What's the point of the first action in double-acting baking powder?
The first "action" is when it gets wet. This is the traditional baking soda + vinegar fizzing. The second rise in double-acting baking powder is when it gets hot. 

In a baked item, such as a cake, this reaction to mosture helps create rise immediately, before the cake starts to change structure, solidify, and form a crust. This enables the center to begin to rise immediately. Without this initial rise, the outside of the cake could have already started to form a crust before the inside is heated enough to rise. This would result in "trapping" the center--either causing a denser unrisen center, or breaking/cracking the crust as it rises and escapes the crust.