State the function of amylopectin in plant cells. Give a reason why it is needed.

Amylopectin is a chain of glucose sugars, or a starch chain. This kind of molecule is known as a polysaccharide, and is component of starch. 
Glucose, as you most likely know, is an easy way for plants to store energy in a way they can use. This simple sugar, however, can be a pain to store on its own, so plant cells use an organelle called the amyloplast to combine many glucose molecules into much more compact amylopectin molecule chains. The amyloplast will build and store these molecules until the plant needs them, at which point the plant will destruct the molecules back into the component glucose molecules. 
The whole point is to allow easy storage of energy stored by photosynthesis, because if energy isn't stored, plants will die at night.