e.currentTarget would always return the component onto which the event listener is added.
On the other hand, e.target can be the component itself or any direct child or grand child or grand-grand-child and so on who received the event. In other words, e.target returns the component which is on top in the Display List hierarchy and must be in the child hierarchy or the component itself.
One use can be when you have several Image in Canvas and you want to drag Images inside the component but Canvas. You can add a listener on Canvas and in that listener you can write the following code to make sure that Canvas wouldn't get dragged.
function dragImageOnly(e:MouseEvent):void
{
if(e.target==e.currentTarget)
{
return;
}
else
{
Image(e.target).startDrag();
}
}