The elements of the ArrayCollection can be used in bindings that will be continued to be monitored. This is not the case with the normal Array class, once the element from an array is used in a binding, it is no longer monitored. ArrayCollection provides a rich set of tools for data manipulation
According to the Adobe flex liveDocs ArrayCollection class is a wrapper class that exposes an Array as a collection that can be accessed and manipulated using the methods and properties of the ICollectionView or IList interfaces. Operations on a ArrayCollection instance modify the data source; for example, if you use the removeItemAt() method on an ArrayCollection, you remove the item from the underlying Array.