Why does java.util.Arrays.sort(Object[]) use 2 kinds of sorting algorithms?

I found that java.util.Arrays.sort(Object[]) use 2 kinds of sorting algorithms(in JDK 1.6).

pseudocode:

if(array.length<7)
   insertionSort(array);
else
   mergeSort(array);

Why does it need 2 kinds of sorting here? for efficiency?